One thing this could be is that the Phone has changed the IP Address on the Router and therefore it no longer has the correct ports forwarded to the Phone (Firewall).

If you get one-way audio, or cannot get a dial tone or cannot make/receive calls with your VoIP device, you are probably behind a firewall. 

Your routers firewall (also known as NAT) is blocking certain operations of the VoIP device.

Before trying to resolve this issue on your router, try the following solutions on your VoIP device configuration. 

Please make changes one at a time and reboot your device each time to see if the problem is solved.

1. Change the SIP registration port on your VoIP device to 5060 or 5065. 

2. Reboot your device and check if you have dial tone.

3. Try adding a STUN server by adding into the STUN Server field on the set up of your VoIP line.