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Inside your router, there should be an option for port triggering. Inside of this area, open up port ranges 5060-5080 and 10000-30000. If you have the option, open them up in UDP protocol. The way these phones work, it should not create a security risk. If you do not have a port triggering option, you can set static IP addresses for the phones through the webpage interface, and then use the static IP addresses for port forwarding. The specific walk-through for your router can vary. You may want to do some quick research on the router to find these functions. This should allow for better data traffic, and may help correct the issue.
If you are unable to find or fix these settings, please fill out as much of the information below as possible.