SIP Capture attached for two register attempts, one on UDP and the other on TCP.
UDP = U 2001:8004:13c0:5fed:dd64:812e:f2cd:8bf6:45417 -> fd00:db8:deca::4:5060 #2718 REGISTER sip:voip.domain.com SIP/2.0. Via: SIP/2.0/UDP 32.1.128.4:45417;rport;branch=z9hG4bKPjZ5fpsipsncRX35bosd-juQhhswwf2n.t. Contact: sip:83@32.1.128.4:45417;ob.
TCP (Works) = T 2001:8004:13c0:5fed:dd64:812e:f2cd:8bf6:63053 -> fd00:db8:deca::4:5060 [AP] #4321 REGISTER sip:voip.domain.com;transport=tcp SIP/2.0. Via: SIP/2.0/TCP [2001:8004:13c0:5fed:dd64:812e:f2cd:8bf6]:63053;rport;branch=z9hG4bKPjZyp0v51VSpi72vDLN0qA-rZBJv4FuMfG;alias. Contact: sip:83@[2001:8004:13c0:5fed:dd64:812e:f2cd:8bf6]:63053;transport=TCP;ob;reg-id=1;+sip.instance="urn:uuid:00000000-0000-0000-0000-0000f9855a1c".
[SIP Capture.txt](https://github.com/kamailio/kamailio/files/6555068/SIP.Capture.txt)