Request-line тоже меняется см ниже (во вложении скрин настройки), я напомню я не использую динамическую регистрация, идентификация происходит по From: 
74956207068@192.168.31.8Session Initiation Protocol (INVITE)
    Request-Line: INVITE sip:302@192.168.31.8 SIP/2.0
    Message Header
        Via: SIP/2.0/UDP 192.168.31.8;rport;branch=z9hG4bK3vp4BKg9gHcUS
        Max-Forwards: 70
        From: "102" <sip:74956207068@192.168.31.8>;tag=ZHSN1tXUXZeyF
        To: <sip:302@192.168.31.8>
        Call-ID: 5049dce8-6d02-1234-aea3-a8f94b03a44e
        CSeq: 79844 INVITE
        Contact: <sip:74956207068@192.168.31.8:5060>
        User-Agent: TAU-8.IP-W/2.0.0 SN/VI09000229 sofia-sip/1.12.10
        Allow: INVITE, ACK, BYE, CANCEL, OPTIONS, PRACK, MESSAGE, SUBSCRIBE, NOTIFY, REFER, UPDATE, INFO
        Supported: timer, 100rel, replaces
        Session-Expires: 1800
        Min-SE: 120
        Content-Type: application/sdp
        Content-Disposition: session
        Content-Length: 256
    Message Body
        Session Description Protocol