Reboot phones: Unplug the A/C adapter or if you have Power over Ethernet switching. Unplug the Ethernet line to the internet port on the underside of the phone. You should see the phone power off. Then plug back in and watch it power on. Make sure you see your extension number and your line keys.Check dial tone, do a test call to a cell or other outside line.
Reboot network switches, router/Firewall & internet modem.
Make sure all equipment is back online. Test your internet to a computer and or phone. If you still have no service or have other issues. Contact support.