Hi When ACK is received on Kamailio from PBX t_check_trans() fails. 200 OK sent PBX from Teams and ACK from PBX is provided here. What could be the issue. SIP/2.0 200 OK FROM: "Ext1"sip:1102@PBX_IP;tag=e08f0a03-1120-4e13-b0e2-90d1a027c05f TO: sip:1101@KAM_IP;tag=e667397cb47c457a860222931e320b41 CSEQ: 25772 INVITE CALL-ID: a6685754-b402-4a15-82b2-5fb2551e5b58 VIA: SIP/2.0/UDP PBX_IP:7790;received=PBX_IP;branch=z9hG4bKPj170912fc-0908-40bb-8248-9f6cb560332d;rport=7790 RECORD-ROUTE: sip:sip-du-a-us.pstnhub.microsoft.com:5061;transport=tls;lr,sip:sbc.callsprout.com;transport=tls;ftag=e08f0a03-1120-4e13-b0e2-90d1a027c05f;lr,sip:KAM_IP:7790;transport=udp;ftag=e08f0a03-1120-4e13-b0e2-90d1a027c05f;lr CONTACT: sip:api-du-b-usea.pstnhub.microsoft.com:443;x-i=c825abba-7203-442c-8461-994522d14365;x-c=25f3c2ce3a635971b809dc3be872e51b/s/1/63c9eea743284db99366cabf5f7dcbf5;ias=52.114.132.46~5061~3 CONTENT-LENGTH: 353 SUPPORTED: timer CONTENT-TYPE: application/sdp ALLOW: INVITE,ACK,OPTIONS,CANCEL,BYE,NOTIFY REQUIRE: timer SESSION-EXPIRES: 1800;refresher=uac SERVER: Microsoft.PSTNHub.SIPProxy v.2020.11.16.5 i.USEA.5 X-Siptrace-Fromip: tls:KAM_IP:7790 X-Siptrace-Toip: udp:PBX_IP:7790 X-Siptrace-Time: 1606205334 607029 X-Siptrace-Method: INVITE X-Siptrace-Dir: out ACK sip:api-du-b-usea.pstnhub.microsoft.com:443;x-i=c825abba-7203-442c-8461-994522d14365;x-c=25f3c2ce3a635971b809dc3be872e51b/s/1/63c9eea743284db99366cabf5f7dcbf5;alias=.114.132.46~5061~3 SIP/2.0 Via: SIP/2.0/UDP PBX_IP:7790;rport;branch=z9hG4bKPjaa30c14c-c965-473d-bc49-9f223f9f634c From: "Ext1" sip:1102@PBX_IP;tag=e08f0a03-1120-4e13-b0e2-90d1a027c05f To: sip:1101@KAM_IP;tag=e667397cb47c457a860222931e320b41 Call-ID: a6685754-b402-4a15-82b2-5fb2551e5b58 CSeq: 25772 ACK Route: sip:KAM_IP:7790;lr;ftag=e08f0a03-1120-4e13-b0e2-90d1a027c05f Route: sip:sbc.callsprout.com;transport=tls;lr;ftag=e08f0a03-1120-4e13-b0e2-90d1a027c05f Route: sip:sip-du-a-us.pstnhub.microsoft.com:5061;transport=tls;lr Max-Forwards: 70 User-Agent: Asterisk Content-Length: 0 X-Siptrace-Fromip: udp:PBX_IP:7790 X-Siptrace-Toip: udp:KAM_IP:7790 X-Siptrace-Time: 1606205334 615495 X-Siptrace-Method: ACK X-Siptrace-Dir: in