Grandstream Remote extensions

Set Account Settings on phone to Register Phones

• Accounts>Account 1> 
• Account Name>  
• Sip Server>
• Sip User ID>
• Authenticate ID >
• Authenticate Password>
• Name>
• Voice Mail Access Number>
• Save and Apply

On the GXP2616 WebUI >

• Account > Audio settings > Codecs PCMU - PCMA -PCMU - PCMA -PCMU - PCMA -PCMU - PCMA 
• Account > Network > NAT > Select STUN
  Settings > General settings > STUN Server : stun.ipvideotalk.com
  Network > Basic > DNS 8.8.8.8 then 8.8.4.4 and 8.8.8.8
  Settings > General settings > Random port: No
• 
  

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

Remote extension guide > http://www.grandstream.com/sites/default/files/Resources/ucm6xxx_remote_extension_configuration_guide_0.pdf

faq's