I'm running Kamailio 5.2.0, whenever I relay an invite via Kamailio, my original contact header is changed from the original:

sip:+XXXXXXXXX@YYY.YYY.YYY.YYYY:5060;transport=udp;gw=netvision

To

sip:YYY.YYY.YYY.YYY;line=sr-.n274i0TMfsHMcCAlVyAMV5IlgCXvV8n4edQlgIUlgKQ4Vk74gI1LmZ69NM79FZAR3JC.cDNLfHUj3Wnp3MP9nd*.

Even when I set it correctly on my route, using:

remove_hf("Contact");

append_hf("Contact: sip:$tU@YYY.YYY.YYY.YYY:5060;transport=udp;gw=netvision\r\n", "Contact");

It still ends up being modified. 

What can I do to keep the contact header as it is?

U YYY.YYY.YYY.YYY:5060 -> 81.24.193.248:5060

INVITE sip:+442033202609@81.24.193.248:5060 SIP/2.0.

Record-Route: <sip:YYY.YYY.YYY.YYY;r2=on;lr=on;ftag=mp0S9yH11vryH;vsf=AAAAAAAAAAAAAAAAAAAAAAADCAEACRgPGB8AAy4xNjM->.

Record-Route: <sip:2YYY.YYY.YYY.YYY;line=sr-.n274V8TlidQMVyUlg87vV8nlfDXlVHB9VD0.VHB9VDoLGtNRbH7ltlTEkyQl3jXEky1LNMoRkt5ukt5ukt5ukt5ukt5ukt5ukt5ukt5W8M5WktckoLuWAsduktTMm64pCAD>.

Via: SIP/2.0/UDP YYY.YYY.YYY.YYY;branch=z9hG4bK53e9.308596b23ed683368534c9d609dce0f6.0.

Via: SIP/2.0/UDP YYY.YYY.YYY.YYY;branch=z9hG4bKsr-kA2uvfsUlerJWtIylgCXvV8n4edQlgIUlgKQ4Vk74gI1.oJVjb2njburlgCXvV8n4edQlgIUlgKQ4HWsRbWPLVDX.GrXLcAHlcC74nZXKbTVpcHh4b6mMGZvJgLsLcJTkgtGu2PGgy**.

Max-Forwards: 67.

From: +18702935016 <sip:+18702935016@YYY.YYY.YYY.YYY>;tag=mp0S9yH11vryH.

To: <sip:+442033202609@81.24.193.248>.

Call-ID: !!:jbkfMgyHjbsDjblfjqAQlVldvgyHKolDlcIHlcknKVIdjcIT.

CSeq: 18182498 INVITE.

Contact: <sip:YYY.YYY.YYY.YYY;line=sr-.n274i0TMfsHMcCAlVyAMV5IlgCXvV8n4edQlgIUlgKQ4Vk74gI1LmZ69NM79FZAR3JC.cDNLfHUj3Wnp3MP9nd*>.

User-Agent: FreeSWITCH-mod_sofia/1.9.0+git~20180706T160334Z~de3df8dc0e~64bit.

Allow: INVITE, ACK, BYE, CANCEL, OPTIONS, MESSAGE, INFO, UPDATE, REGISTER, REFER, NOTIFY.

Supported: timer, path, replaces.

Allow-Events: talk, hold, conference, refer.

Content-Type: application/sdp.

Content-Disposition: session.

Content-Length: 224.

Remote-Party-ID: "+18702935016" <sip:+18702935016@YYY.YYY.YYY.YYY>;party=calling;screen=yes;privacy=off.

.

v=0.

o=FreeSWITCH 1585439298 1585439299 IN IP4 ZZZ.ZZZ.ZZZ.ZZZ.

s=FreeSWITCH.