Set Account Settings on phone to Register Phones
On the GXP2616 WebUI >
After those Reboot the phone and confirmed working on session and call.
Make port forward on the router side for protocols TCP/UDP
If you use firewall or router, check to forward the following ports to communicate with UCM server (On the router or firewall )
5060 to 5070 for SIP communications, (UDP/TCP)
5061 for TLS,
10000 - 20000 for RTP