I shorten the trace to (hopefully) the relevant things. Asterisk is on 192.168.10.72, port 25060, proxy is OpenSIPS on the same machine with port 5060, in this case there are two phones, 192.168.10.9 and 192.168.10.221. Asterisk => Proxy: U 192.168.10.72:25060 -> 192.168.10.72:5060 INVITE sip:arthur@192.168.10.72 SIP/2.0. Via: SIP/2.0/UDP 192.168.10.72:25060;branch=z9hG4bK7b4dc098. Max-Forwards: 70. From: "0100" ;tag=as0ae41e2c. To: . Contact: . Call-ID: 7c6581e95fcfe68e67622cf95fc15f71@192.168.10.72. CSeq: 102 INVITE. ... sdp cut of ... Proxy => Asterisk U 192.168.10.72:5060 -> 192.168.10.72:25060 SIP/2.0 100 Giving a try. Via: SIP/2.0/UDP 192.168.10.72:25060;branch=z9hG4bK7b4dc098. From: "0100" ;tag=as0ae41e2c. To: . Call-ID: 7c6581e95fcfe68e67622cf95fc15f71@192.168.10.72. CSeq: 102 INVITE. Server: OpenSIPS (1.6.4-2-notls (i386/linux)). Proxy => phone1 U 192.168.10.72:5060 -> 192.168.10.221:34381 INVITE sip:arthur@192.168.10.221:34381;line=gf4fq202 SIP/2.0. Via: SIP/2.0/UDP 192.168.10.72;branch=z9hG4bK4b49.0d5ce661.0. Via: SIP/2.0/UDP 192.168.10.72:25060;branch=z9hG4bK7b4dc098. Max-Forwards: 69. From: "0100" ;tag=as0ae41e2c. To: . Contact: . Call-ID: 7c6581e95fcfe68e67622cf95fc15f71@192.168.10.72. CSeq: 102 INVITE. ... sdp cut of ... Proxy => phone1 U 192.168.10.72:5060 -> 192.168.10.9:5060 INVITE sip:arthur@192.168.10.9 SIP/2.0. Via: SIP/2.0/UDP 192.168.10.72;branch=z9hG4bK4b49.0d5ce661.1. Via: SIP/2.0/UDP 192.168.10.72:25060;branch=z9hG4bK7b4dc098. Max-Forwards: 69. From: "0100" ;tag=as0ae41e2c. To: . Contact: . Call-ID: 7c6581e95fcfe68e67622cf95fc15f71@192.168.10.72. CSeq: 102 INVITE. ... sdp cut of ... phone1 => Proxy U 192.168.10.9:5060 -> 192.168.10.72:5060 SIP/2.0 100 Trying. Via: SIP/2.0/UDP 192.168.10.72;branch=z9hG4bK4b49.0d5ce661.1,SIP/2.0/UDP 192.168.10.72:25060;branch=z9hG4bK7b4dc098. To: . From: "0100" ;tag=as0ae41e2c. Call-ID: 7c6581e95fcfe68e67622cf95fc15f71@192.168.10.72. CSeq: 102 INVITE. Server: Twinkle/1.4.2. phone1 => Proxy U 192.168.10.9:5060 -> 192.168.10.72:5060 SIP/2.0 180 Ringing. Via: SIP/2.0/UDP 192.168.10.72;branch=z9hG4bK4b49.0d5ce661.1,SIP/2.0/UDP 192.168.10.72:25060;branch=z9hG4bK7b4dc098. To: ;tag=plkqt. From: "0100" ;tag=as0ae41e2c. Call-ID: 7c6581e95fcfe68e67622cf95fc15f71@192.168.10.72. CSeq: 102 INVITE. Contact: . Server: Twinkle/1.4.2. Proxy => Asterisk U 192.168.10.72:5060 -> 192.168.10.72:25060 SIP/2.0 180 Ringing. Via: SIP/2.0/UDP 192.168.10.72:25060;branch=z9hG4bK7b4dc098. To: ;tag=plkqt. From: "0100" ;tag=as0ae41e2c. Call-ID: 7c6581e95fcfe68e67622cf95fc15f71@192.168.10.72. CSeq: 102 INVITE. Contact: . Server: Twinkle/1.4.2. phone2 => Proxy U 192.168.10.221:34381 -> 192.168.10.72:5060 SIP/2.0 180 Ringing. Via: SIP/2.0/UDP 192.168.10.72;branch=z9hG4bK4b49.0d5ce661.0. Via: SIP/2.0/UDP 192.168.10.72:25060;branch=z9hG4bK7b4dc098. From: "0100" ;tag=as0ae41e2c. To: ;tag=ijb4out3fh. Call-ID: 7c6581e95fcfe68e67622cf95fc15f71@192.168.10.72. CSeq: 102 INVITE. Contact: ;reg-id=1. Proxy => Asterisk U 192.168.10.72:5060 -> 192.168.10.72:25060 SIP/2.0 180 Ringing. Via: SIP/2.0/UDP 192.168.10.72:25060;branch=z9hG4bK7b4dc098. From: "0100" ;tag=as0ae41e2c. To: ;tag=ijb4out3fh. Call-ID: 7c6581e95fcfe68e67622cf95fc15f71@192.168.10.72. CSeq: 102 INVITE. Contact: ;reg-id=1. phone2 => Proxy U 192.168.10.221:34381 -> 192.168.10.72:5060 SIP/2.0 180 Ringing. Via: SIP/2.0/UDP 192.168.10.72;branch=z9hG4bK4b49.0d5ce661.0. Via: SIP/2.0/UDP 192.168.10.72:25060;branch=z9hG4bK7b4dc098. From: "0100" ;tag=as0ae41e2c. To: ;tag=ijb4out3fh. Call-ID: 7c6581e95fcfe68e67622cf95fc15f71@192.168.10.72. CSeq: 102 INVITE. Contact: ;reg-id=1. Proxy => Asterisk U 192.168.10.72:5060 -> 192.168.10.72:25060 SIP/2.0 180 Ringing. Via: SIP/2.0/UDP 192.168.10.72:25060;branch=z9hG4bK7b4dc098. From: "0100" ;tag=as0ae41e2c. To: ;tag=ijb4out3fh. Call-ID: 7c6581e95fcfe68e67622cf95fc15f71@192.168.10.72. CSeq: 102 INVITE. Contact: ;reg-id=1. phone2 => Proxy, Phone2 answeres call U 192.168.10.221:34381 -> 192.168.10.72:5060 SIP/2.0 200 Ok. Via: SIP/2.0/UDP 192.168.10.72;branch=z9hG4bK4b49.0d5ce661.0. Via: SIP/2.0/UDP 192.168.10.72:25060;branch=z9hG4bK7b4dc098. From: "0100" ;tag=as0ae41e2c. To: ;tag=ijb4out3fh. Call-ID: 7c6581e95fcfe68e67622cf95fc15f71@192.168.10.72. CSeq: 102 INVITE. Contact: ;reg-id=1. User-Agent: snom720/8.7.3.201201302239. ... sdp cut of ... Proxy => Asterisk U 192.168.10.72:5060 -> 192.168.10.72:25060 SIP/2.0 200 Ok. Via: SIP/2.0/UDP 192.168.10.72:25060;branch=z9hG4bK7b4dc098. From: "0100" ;tag=as0ae41e2c. To: ;tag=ijb4out3fh. Call-ID: 7c6581e95fcfe68e67622cf95fc15f71@192.168.10.72. CSeq: 102 INVITE. Contact: ;reg-id=1. User-Agent: snom720/8.7.3.201201302239. ... sdp cut of ... Asterisk => phone1 (The ACK is sent to phone1 instead of phone2!) U 192.168.10.72:25060 -> 192.168.10.9:5060 ACK sip:arthur@192.168.10.9 SIP/2.0. Via: SIP/2.0/UDP 192.168.10.72:25060;branch=z9hG4bK37d8b493. Max-Forwards: 70. From: "0100" ;tag=as0ae41e2c. To: ;tag=ijb4out3fh. Contact: . Call-ID: 7c6581e95fcfe68e67622cf95fc15f71@192.168.10.72. CSeq: 102 ACK. Proxy => phone1, Cancel call as phone2 has connected U 192.168.10.72:5060 -> 192.168.10.9:5060 CANCEL sip:arthur@192.168.10.9 SIP/2.0. Via: SIP/2.0/UDP 192.168.10.72;branch=z9hG4bK4b49.0d5ce661.1. From: "0100" ;tag=as0ae41e2c. Call-ID: 7c6581e95fcfe68e67622cf95fc15f71@192.168.10.72. To: . CSeq: 102 CANCEL. Max-Forwards: 70. Reason: SIP ;cause=200 ;text="Call completed elsewhere". User-Agent: OpenSIPS (1.6.4-2-notls (i386/linux)). phone1 => Proxy U 192.168.10.9:5060 -> 192.168.10.72:5060 SIP/2.0 200 OK. Via: SIP/2.0/UDP 192.168.10.72;branch=z9hG4bK4b49.0d5ce661.1. To: ;tag=plkqt. From: "0100" ;tag=as0ae41e2c. Call-ID: 7c6581e95fcfe68e67622cf95fc15f71@192.168.10.72. CSeq: 102 CANCEL. Server: Twinkle/1.4.2. phone1 => Proxy U 192.168.10.9:5060 -> 192.168.10.72:5060 SIP/2.0 487 Request Terminated. Via: SIP/2.0/UDP 192.168.10.72;branch=z9hG4bK4b49.0d5ce661.1,SIP/2.0/UDP 192.168.10.72:25060;branch=z9hG4bK7b4dc098. To: ;tag=plkqt. From: "0100" ;tag=as0ae41e2c. Call-ID: 7c6581e95fcfe68e67622cf95fc15f71@192.168.10.72. CSeq: 102 INVITE. Server: Twinkle/1.4.2. Proxy => phone1 U 192.168.10.72:5060 -> 192.168.10.9:5060 ACK sip:arthur@192.168.10.9 SIP/2.0. Via: SIP/2.0/UDP 192.168.10.72;branch=z9hG4bK4b49.0d5ce661.1. From: "0100" ;tag=as0ae41e2c. Call-ID: 7c6581e95fcfe68e67622cf95fc15f71@192.168.10.72. To: ;tag=plkqt. CSeq: 102 ACK. Max-Forwards: 70. User-Agent: OpenSIPS (1.6.4-2-notls (i386/linux)). phone2 => Proxy: Repeat 200 OK, as there was no ACK! U 192.168.10.221:34381 -> 192.168.10.72:5060 SIP/2.0 200 Ok. Via: SIP/2.0/UDP 192.168.10.72;branch=z9hG4bK4b49.0d5ce661.0. Via: SIP/2.0/UDP 192.168.10.72:25060;branch=z9hG4bK7b4dc098. From: "0100" ;tag=as0ae41e2c. To: ;tag=ijb4out3fh. Call-ID: 7c6581e95fcfe68e67622cf95fc15f71@192.168.10.72. CSeq: 102 INVITE. Contact: ;reg-id=1. User-Agent: snom720/8.7.3.201201302239. ... sdp cut of ... Proxy => Asterisk U 192.168.10.72:5060 -> 192.168.10.72:25060 SIP/2.0 200 Ok. Via: SIP/2.0/UDP 192.168.10.72:25060;branch=z9hG4bK7b4dc098. From: "0100" ;tag=as0ae41e2c. To: ;tag=ijb4out3fh. Call-ID: 7c6581e95fcfe68e67622cf95fc15f71@192.168.10.72. CSeq: 102 INVITE. Contact: ;reg-id=1. User-Agent: snom720/8.7.3.201201302239. Allow: INVITE, ACK, CANCEL, BYE, REFER, OPTIONS, NOTIFY, SUBSCRIBE, PRACK, MESSAGE, INFO, UPDATE. Allow-Events: talk, hold, refer, call-info. Supported: timer, 100rel, replaces, from-change. Content-Type: application/sdp. Content-Length: 246. ... sdp cut of ... Asterisk => phone1: ACK to wrong phone! U 192.168.10.72:25060 -> 192.168.10.9:5060 ACK sip:arthur@192.168.10.9 SIP/2.0. Via: SIP/2.0/UDP 192.168.10.72:25060;branch=z9hG4bK765edd54. Max-Forwards: 70. From: "0100" ;tag=as0ae41e2c. To: ;tag=ijb4out3fh. Contact: . Call-ID: 7c6581e95fcfe68e67622cf95fc15f71@192.168.10.72. CSeq: 102 ACK. This repeats up to 30 Seconds until phone2 gives up: phone2 => Proxy: Repeat 200 OK, as there was no ACK! Proxy => Asterisk Asterisk => phone1: ACK to wrong phone! After about 30 seconds: phone2 => Asterisk: Giving up U 192.168.10.221:34381 -> 192.168.10.72:25060 BYE sip:0100@192.168.10.72:25060 SIP/2.0. Via: SIP/2.0/UDP 192.168.10.221:34381;branch=z9hG4bK-z69ibilvoo3g;rport. From: ;tag=ijb4out3fh. To: "0100" ;tag=as0ae41e2c. Call-ID: 7c6581e95fcfe68e67622cf95fc15f71@192.168.10.72. CSeq: 1 BYE. Max-Forwards: 70. Contact: ;reg-id=1. User-Agent: snom720/8.7.3.201201302239. Asterisk => phone2 U 192.168.10.72:25060 -> 192.168.10.221:34381 SIP/2.0 200 OK. Via: SIP/2.0/UDP 192.168.10.221:34381;branch=z9hG4bK-z69ibilvoo3g;received=192.168.10.221;rport=34381. From: ;tag=ijb4out3fh. To: "0100" ;tag=as0ae41e2c. Call-ID: 7c6581e95fcfe68e67622cf95fc15f71@192.168.10.72. CSeq: 1 BYE.