From the Nextiva voice admin dashboard, hover over Users > Manage Users > select the user > Mobility > SimultaneousRing.
Enter the phone number. Choose to enable Answerconfirmation so unanswered calls route to an appropriate destination, such as your Nextiva voicemail and not your personal voicemail. Save.
You can also set up criteria based on a specified schedule and phone number(s), including any private or unavailable numbers. Click the Plus (+) icon under the simultaneous ring menu and enter the required information > SaveCriteria.