What Is a SIP Phone & How Does It Work?

October 6, 2023 12 min read

Jeremiah Zerby

Jeremiah Zerby

What is a SIP Phone?

As you research phones for the office, you’ll come across a type of phone known as an IP phone. These devices are also known as SIP phones. SIP phones aren’t anything like analog-based antiques of the past  — they work exclusively over the internet. 

In this guide, we’ll explain exactly what a SIP phone is, how it works to make phone calls, and which models are likely to work best for you. 

But first, let’s explain a couple of quick definitions and meanings around these IP phones.

What Is a SIP Phone?

A SIP phone is a device that uses the internet to make and receive phone calls. Instead of relying on traditional phone lines, SIP phones use a technology called Voice over IP (VoIP) to transmit voice data over the internet. This allows for a number of benefits over traditional phones, such as lower costs, advanced features, and wideband audio (HD voice). 

There are two types of SIP phones: hardware and software. 

Examples of SIP applications are found in many communication products today.

SIP technology is found in conference phones, softphones, and even meeting software, which all use SIP protocol. Many organizations have moved towards retrofitting their IP PBX systems with SIP trunk provider to lower costs and add unlimited capacity.

What Is SIP? 

SIP is short for Session Initiation Protocol, which is used to establish real-time voice, video, and messaging between two or more devices. It’s essentially the foundation of internet-based phone service, commonly known as VoIP. 

Session Initiation Protocol (SIP) is a communication protocol that manages multimedia communication, such as video and voice calls. To use SIP, you need a SIP phone that establishes communication over the internet. 

Unlike traditional phone systems, SIP phones rely on Internet technology to make secure and reliable calls. The SIP protocol enables organizations to have unified communications as it integrates basic phone capabilities with video, email, instant messaging, and more. Organizations use SIP phones if they want scalability, reliability, and unlimited voice calls. 

Typical SIP Phone Features

SIP phones may resemble their predecessors, but their inner workings have substantially more functionality than traditional phones. Unlike their landline counterparts, SIP phones easily manage features like call hold, transfer to extensions, and even conference calling. 

For IT professionals, the appeal lies in the simplified infrastructure. SIP phones connect directly to VoIP service providers, eliminating the need for additional hardware or server setups. This translates to lower maintenance costs compared to traditional phone systems. 

Let’s look at the popular features of a SIP phone. 

Top features of a SIP phone:

Differences between VoIP vs. SIP

While often used interchangeably, Voice over Internet Protocol (VoIP) and Session Initiation Protocol (SIP) play distinct roles in business communications.

VoIP forms the technological backbone, enabling voice communication to travel over broadband connections instead of traditional phone lines. It digitizes voice data, resulting in clearer call quality and opening doors to features like video conferencing and call recording. With a VoIP provider, calls can connect between the public switched telephone network (PSTN). 

SIP, on the other hand, acts as a standardized language. It dictates how devices and platforms initiate, manage, and terminate communication sessions between users. By establishing a common set of rules, SIP ensures seamless interoperability between diverse VoIP systems, allowing calls to flow smoothly across different devices and platforms.

In essence, VoIP provides the calling technology, while SIP acts as the standardized communication protocol that makes it all work seamlessly. While VoIP can function without SIP, incorporating this universally accepted language maximizes compatibility and ensures a smooth communication experience.

Types of SIP phones

SIP phones generally fall into two categories: hard phones and softphones. SIP phones can only work with cloud-based phone systems, unlike traditional analog phones.

Hard Phones (IP desk phones and conference phones)

A hard phone looks like a regular telephone and, indeed, behaves like one. Hard phones connect using Ethernet cables and can come in a cordless form. Popular SIP hard phones include Poly, Cisco, Nextiva, and Panasonic. Most of these desk phones offer similar features, with a few differences between them.

Not all hard phones are the same. They are classified into basic, intermediate, and advanced categories.

  • Basic: Considered almost a starter VoIP phone, these SIP phones are what you might see used by call center or contact center staff. These phones have a dial pad and can complete internal and external phone calls. These hard phones are budget-friendly and are very easy to use.
  • Intermediate: A step up from basic, intermediate SIP hard phones provide more robust functionality, often including a browsable directory with multiple extensions. Physically, these phones might have a few more buttons for extra VoIP features. These phones are often deployed across the workforce, even sent with employees if they work from home.
  • Advanced: Advanced SIP hard phones can do anything a basic or intermediate phone can do and often feature a full-color display, multiple extensions, Wi-Fi or Bluetooth compatibility. Visually, these phones look more refined and are enjoyed by company executives. These phones may have fewer physical buttons because applications can be accessed through the display.

One limitation of hard phones is that they are stationary. Over half of the United States workforce is predicted to work remotely over the coming years, and businesses are looking into low-cost alternatives for desk phones. This is where softphones come into play.

Related: What Is a Contact Center? Definition, Features, and Uses

Softphones (VoIP apps & contact center software)

Softphones are applications that provide users with phone service connectivity through software installed on a desktop, laptop, or smartphone. Once your VoIP app is installed, use a headset and microphone for the best calling experience. 

A softphone may be installed by administrators or by users in a self-service format. IT leaders who are considering using softphone technology need to think about their network utilization. SIP traffic from softphones can’t be easily singled out as compared to other network applications. For those with a mandatory VPN, you will want to complete a VoIP line test to evaluate the health of your connection. 

With VoIP, call quality adapts to changing network conditions automatically, such as limited network bandwidth. For frequent softphone users, they might experience shorter battery life on their cell phones. These factors mean that IT leaders have to plan ahead before deploying softphone apps to their employees. 

Related: VoIP Basics: The Beginner’s Guide to Mastering VoIP

How SIP-based telephony works

Session Initiation Protocol-based telephony employs straightforward commands akin to those used to access web pages. The analog signals from the handset are digitized and transmitted as data packets through the network to the call recipient. 

Hosted VoIP Phone System Network Diagram

SIP phones connect to servers which initiate contacts with other extensions or route calls. When a call connects, the server and phones negotiate call quality and codecs in real-time.  Here’s technical overview broken down in six steps. 

How SIP calls work

  1. A user picks up the handset of a SIP phone and dials a number to place a call.
  2. The SIP phone sends a SIP “invite” message to the SIP server it is registered with. This message indicates that the user wants to initiate a call.
  3. The SIP server processes the invite request. It may connect the call to another extension registered to the same SIP server, or it may route the call out over the Public Switched Telephone Network to connect the party.
  4. Once the dialed party answers, the two SIP phones exchange information to set up two-way audio transmission. They agree on which codecs and bandwidth to use.
  5. As the call progresses, the phones continue to send SIP signaling messages to provide status updates and ensure quality. For example, if packet loss occurs, a phone may request a codec with lower bandwidth requirements.
  6. When one party hangs up, a SIP “bye” message is sent to terminate the call on both sides and disconnect the media session.
SIP Protocol Diagram

The good news here is that nearly all users and most IT administrators won’t ever need to deal with any of the minutiae of the SIP protocol. With a reliable business VoIP platform like Nextiva, you’ll focus on assigning virtual phone numbers and extensions to users and adjusting settings.

Benefits of SIP phones

It can be very cost-effective to upgrade to SIP phones, no matter the size of your company. Overall, there are several advantages to using SIP phones.

Your main use for SIP phones should be identified more than the brand of SIP phone you select. Think through the real-world examples of how your employees use their phones to communicate. 

Some employees need more powerful phones, like a busy receptionist or a prominent business executive. Even for small businesses, there’s an affordable SIP phone that will help your team communicate more effectively. 

Top-Rated Business SIP Phones

To help you through your next telephony technology investment, we have put together a quick comparison guide of the top SIP phones available on the market today. Just a few notes to keep in mind:

  • Mix and match them to your employee’s needs
  • Support and configuration are often included from a SIP provider
  • You have the option of purchasing or leasing (renting)

Basic SIP Phones

Poly Edge E220

Poly Edge E220 SIP Phone

The Poly Edge E220 phone will keep you sounding professional. This SIP phone pairs the famous Poly noise-blocking technology with a cutting-edge design that brings style to any desktop or reception area. The Poly Edge E220 phone is always on, easy to use, secure, reliable, and stable. Priced at $160.

  • Number of Lines: 4
  • Display Type: Color LCD
  • Screen Size: 2.8″
  • POE Available: Yes
  • Bluetooth: Yes
  • Wi-Fi: No
  • USB port: 1 USB Type C

Yealink T33G

Yealink T33G SIP Phone

An entry-level color screen IP phone with high performance, the Yealink SIP-T33G offers support for four lines. For its fashionable appearance as well as a 320×240-pixel color display with backlight, it brings a comfortable operation experience and clear visual experience for users. Priced at $160.

  • Number of Lines: 4
  • Display Type: Color
  • Screen Size: 2.4″
  • POE Available: Yes
  • Headset Input: RJ-9/EHS

Intermediate SIP Phones

Nextiva X-815

Nextiva X-815 IP Phone

The Nextiva X-815 is a sleek, six-line, color, SIP phone ideal for call center or office workers. The phone’s interface is clean and well-organized, so you’ll handle calls with greater ease and confidence. Priced at $130.

  • Number of Lines: 4
  • Display Type: Color LCD
  • Screen Size: 2.8″
  • POE Available: Yes
  • Gig Ethernet Ports: Yes
  • Wi-Fi: Yes, USB A210 Adapter Required (sold separately)
  • Speed Dialing: Yes
  • Headset Input: RJ-9
  • USB port: 1

Poly VVX 350

Poly VVX 350 SIP Phone

A high-quality, six-line, color, mid-range, SIP phone ideal for call center operators who handle moderate/high volume of calls and for small teams to know which team members are on the phone. Priced at $220.

  • Number of Lines: 6 native lines (24 with pagination)
  • Display Type: Color LCD
  • Screen Size: 3.5″
  • PoE Available: Yes
  • Headset Input: RJ-9
  • USB ports: 2
  • Bluetooth/Wi-Fi: Yes, both via dongle
  • Warranty: 1 year; lifetime when rented

Advanced SIP Phones

Nextiva X-885

Nextiva X-885 SIP Phone

A sleek, twelve-line, color, SIP desk phone ideal for executives and receptionists. It has a multi-page approach to provide twelve additional programmable keys. It comes preconfigured out of the box for Nextiva’s business phone system. 

  • Number of Lines: 12
  • Display Type: Color LCD
  • Screen Size: 4.3″
  • PoE Available: Yes
  • Gig Ethernet Ports: Yes
  • USB port: 1
  • HD Voice: Yes

Cisco IP Phone 8865

Cisco 8865 SIP Phone

The Cisco IP Phone 8865 is a premium enterprise SIP phone designed for executives and managers. It features a large color touchscreen display, Bluetooth support, wired and wireless headset options, a handset, integrated Wi-Fi and Ethernet ports, and up to 10 business phone lines. The high-resolution 5-inch touchscreen enables rich visual communication, while the wideband audio with HD Voice delivers crisp, clear sound quality. Priced at $350.

  • Number of Lines: 10
  • Display Type: Color LCD
  • Screen Size: 5″
  • POE Available: Yes
  • Gig Ethernet Ports: Yes
  • Speed Dialing: Yes
  • Bluetooth: Yes
  • Wi-Fi calling: Yes
  • HD Voice: Yes
  • USB ports: 2
  • Expansion Module Connectivity: up to 3
  • Headset Input: RJ-9

How to Get a Business Phone Number

The process of getting a phone number for your SIP phone begins by signing up with a VoIP provider. You have the option of using your existing phone numbers through a process known as porting. While porting may take a couple of weeks to complete, you can use a virtual number right away. 

Since every SIP phone has a hardware ID assigned, it looks to the service provider to know which numbers it can use. You or your phone service provider can associate SIP phones with any desired phone number that you authorize. 

With the Nextiva VoIP platform, your SIP phone comes configured out of the box. Plug it in, and you can begin taking phone calls right away.

Choosing Your SIP Phone Service Provider

As you decide on the equipment you need for your company’s communications needs, consider these best practices.

Last but certainly not least is the quality of support you have available. Is your VoIP provider available at 9 p.m.? How fast do they answer the phone at 10 a.m.? When it comes to your business, you can’t let life pass you by while waiting on hold.

Now that you’ve researched everything about SIP phones, why not check out VoIP phones we have available. Our team thoroughly tests and supports every device to confirm it meets your business needs.


How does a SIP phone differ from SIP trunking?

SIP phones are the devices you use to make and receive calls, like cordless phones but powered by your internet connection. They connect to a SIP service provider who manages the call routing.

SIP trunking is a service itself, acting like a bridge between your business’s IP PBX system and the PSTN (traditional phone network). It allows your existing IP PBX to make and receive calls through the internet, without needing separate phone lines.

What is a SIP phone service?

A SIP phone service provider is the backbone of your SIP phone system. They provide the infrastructure and software needed to route calls, manage phone numbers, and offer additional features like voicemail, call recording, and conferencing. Primarily, they establish calls between the public telephone network and your internal office phone system.

Consider factors like call quality, pricing, reliability, and desired features. Research and compare to find the best fit for your business needs.

How can I achieve maximum reliability with a SIP phone?

Follow these tips to ensure your SIP phone system delivers reliable and clear communication.

– Use high-speed internet: Maintain a consistent and reliable internet connection to avoid call drops and choppy audio.
– Quality equipment: Invest in good quality SIP phones and network hardware from trusted vendors to minimize technical issues.
– Monitor performance: Track call quality metrics and diagnose any recurring issues early.
– Set up Quality of Service (QoS): Tune your network to prioritize voice traffic so it doesn’t get congested when users share or stream video over the internet.

Jeremiah Zerby


Jeremiah Zerby

Jeremiah Zerby is a marketing specialist at Nextiva. He spent three years on the front lines of technical support, troubleshooting internet and VoIP topics. He moved forward into the technical writing and content creation space. He’s helped set up hundreds of businesses and advised thousands of people with their cloud communications.

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