I am using Linksys PAP2 as an external extension, logged in on my 3cx-system. I can call to this PAP2-phone, for instance from another external softphone connected to the same 3cx-system, all ok. This means the phone is registered. Making a call the other way does not work. I can not call from my PAP2, neither to another internal phone nor to an external phone. The IP-address of the 3cx system is dynamic, so I use dynamic DNS to be able to find my 3cx-system. This works fine, my PAP2 always registers with the 3cx-system. The error message at the 3cx-side is: Unidentified incoming call. Review INVITE and adjust source identification. etc When I, in "Line 1" section in the PAP2, change the Proxy-field from mydynamicdns.XXX (my dynamic DNS placeholder) to the actual IP address of the 3cx system it all works. Below som log-data from my 3cx system, first the call in the working direction, and then a call in opposite direction which fails. Global IP-address 22.214.171.124 and dynamic DNS placeholder mydynamicdns.XXX have been modified for safety-reasons. This call from remote internal 14 (3cx softphone) to remote internal 16 (analog phone on PAP2) works: 23:58:24.375 [CM503008]: Call(64): Call is terminated 23:58:24.359 [CM503008]: Call(64): Call is terminated 23:58:22.625 [CM503007]: Call(64): Device joined: sip:firstname.lastname@example.org:51242 23:58:22.609 [CM503007]: Call(64): Device joined: sip:email@example.com:56196;rinstance=e988eaedb8ffe0df 23:58:14.390 [CM503024]: Call(64): Calling Ext:Ext.16@[Dev:sip:firstname.lastname@example.org:51242] 23:58:14.171 [CM503004]: Call(64): Route 1: Ext:Ext.16@[Dev:sip:email@example.com:51242] 23:58:14.140 [CM503001]: Call(64): Incoming call from Ext.14 to <sip:firstname.lastname@example.org:5060> This call in opposite direction fails: 00:00:21.468 [CM503008]: Call(65): Call is terminated 00:00:21.468 [CM502001]: Source info: From: "16"<sip:16@mydynamicdns.XXX>;tag=2dfbf6c3f613605co0<sip:14@mydynamicdns.XXX> 00:00:21.468 [CM503013]: Call(65): Incoming call rejected, caller is unknown; msg=SipReq: INVITE 14@mydynamicdns.XXX tid=-cb78e2b9 cseq=INVITE email@example.com:51242 / 102 from(wire) 00:00:21.250 [CM500002]: Unidentified incoming call. Review INVITE and adjust source identification: INVITE sip:14@mydynamicdns.XXX SIP/2.0 Via: SIP/2.0/UDP 126.96.36.199:51242;branch=z9hG4bK-2e1c7245;rport=51242 Max-Forwards: 70 Contact: "16"<sip:firstname.lastname@example.org:51242> To: <sip:14@mydynamicdns.XXX> From: "16"<sip:16@mydynamicdns.XXX>;tag=2dfbf6c3f613605co0 Call-ID: email@example.com CSeq: 101 INVITE Expires: 240 Allow: ACK, BYE, CANCEL, INFO, INVITE, NOTIFY, OPTIONS, REFER Supported: x-sipura, replaces User-Agent: Linksys/PAP2T-5.1.6(LS) Content-Length: 0 00:00:21.250 [CM302001]: Authorization system can not identify source of: SipReq: INVITE 14@mydynamicdns.XXX tid=-2e1c7245 cseq=INVITE firstname.lastname@example.org:51242 / 101 from(wire) Hope this might help to find out where the problem lies. Any suggestions?