It seems like this issue is introduced by PCSCF when Soft Phone registers. The SIP Register from PCSCF to ICSCF is as below:
REGISTER sip:ims.mnc001.mcc001.3gppnetwork.org SIP/2.0 Via: SIP/2.0/UDP [FC00:1235:1:0:0:0:0:45];branch=z9hG4bK9fc5.213ad03f122aa3a2b0840eae36a852ba.1 Via: SIP/2.0/UDP [fc00:1235:3:1::]:5060;rport=5060;received=FC00:1235:3:1:0:0:0:0;branch=z9hG4bK2107430722 From: sip:alice@ims.mnc001.mcc001.3gppnetwork.org;tag=60956342 To: sip:alice@ims.mnc001.mcc001.3gppnetwork.org Call-ID: 2069996946 CSeq: 1 REGISTER Contact: sip:alice@[fc00:1235:3:1::];line=572ba75522c0a87;alias=[FC00:1235:3:1:0:0:0:0]~5060~1 Max-Forwards: 69 User-Agent: Linphone/3.6.1 (eXosip2/4.1.0) Expires: 3600 Content-Length: 0 Path: sip:term@pcscf.ims.mnc001.mcc001.3gppnetwork.org:5060;nat=yes;received=**sip:FC00:1235:3:1:0:0:0:0:5060**;lr Supported: path Require: path P-Visited-Network-ID: ims.mnc001.mcc001.3gppnetwork.org
The SIP Register from Soft Phone is fine:
REGISTER sip:ims.mnc001.mcc001.3gppnetwork.org SIP/2.0 Via: SIP/2.0/UDP [fc00:1235:3:1::]:5060;branch=z9hG4bK2107430722 From: sip:alice@ims.mnc001.mcc001.3gppnetwork.org;tag=60956342 To: sip:alice@ims.mnc001.mcc001.3gppnetwork.org Call-ID: 2069996946 CSeq: 1 REGISTER Contact: sip:alice@[fc00:1235:3:1::];line=572ba75522c0a87 Max-Forwards: 70 User-Agent: Linphone/3.6.1 (eXosip2/4.1.0) Expires: 3600 Content-Length: 0