If your phone shows an error message of "SIP registered failed", can't dial out, or is not receiving phone calls unplug the Ethernet cable from the LAN port on the back of your phone wait about 5 seconds and plug it back. You'll know if you unplugged the right cable if your screen is black. 

Wait until it comes back on and try to call someone.

If you do NOT have an error message and you are not receiving phone calls, then check if you are on Do Not Disturb mode. 

If you are still having issues place an ALL IT Ticket.