I even gone through  the data sheet page but I missed this image.. I think I’m getting old..:)Thanks @Ashish Patel

From the Diagram, 7275 extension  is in a different  network and registered with the CME_3945 where 7271 and 7272 is registered .  your problem is SIP phone (3rd party) couldn't receive calls. It can make calls, but I can't hear the other side. if 72...