I shorten the trace to (hopefully) the relevant things. Asterisk is on 192.168.10.72, port 25060, proxy is OpenSIPS on the same machine with port 5060, the phone which is ringing is on 192.168.10.221.

Asterisk => Proxy:
U 192.168.10.72:25060 -> 192.168.10.72:5060
INVITE sip:arthur@192.168.10.72 SIP/2.0.
Via: SIP/2.0/UDP 192.168.10.72:25060;branch=z9hG4bK6c6f4860.
Max-Forwards: 70.
From: "Max M..ller" <sip:245@192.168.10.72>;tag=as3cafd135.
To: <sip:arthur@192.168.10.72>.
Contact: <sip:245@192.168.10.72:25060>.
Call-ID: 4c640ebd11d192e15bfc6d3a667684ce@192.168.10.72.
CSeq: 102 INVITE.
... sdp cut of ...

Proxy => Asterisk
U 192.168.10.72:5060 -> 192.168.10.72:25060
SIP/2.0 100 Giving a try.
Via: SIP/2.0/UDP 192.168.10.72:25060;branch=z9hG4bK6c6f4860.
From: "Max M..ller" <sip:245@192.168.10.72>;tag=as3cafd135.
To: <sip:arthur@192.168.10.72>.
Call-ID: 4c640ebd11d192e15bfc6d3a667684ce@192.168.10.72.
CSeq: 102 INVITE.

Proxy => phone
U 192.168.10.72:5060 -> 192.168.10.221:34381
INVITE sip:arthur@192.168.10.221:34381;line=478vzxb3 SIP/2.0.
Via: SIP/2.0/UDP 192.168.10.72;branch=z9hG4bK24be.5163d992.0.
Via: SIP/2.0/UDP 192.168.10.72:25060;branch=z9hG4bK6c6f4860.
Max-Forwards: 69.
From: "Max M..ller" <sip:245@192.168.10.72>;tag=as3cafd135.
To: <sip:arthur@192.168.10.72>.
Contact: <sip:245@192.168.10.72:25060>.
Call-ID: 4c640ebd11d192e15bfc6d3a667684ce@192.168.10.72.
CSeq: 102 INVITE.
... sdp cut of ...

Phone => Proxy
U 192.168.10.221:34381 -> 192.168.10.72:5060
SIP/2.0 180 Ringing.
Via: SIP/2.0/UDP 192.168.10.72;branch=z9hG4bK24be.5163d992.0.
Via: SIP/2.0/UDP 192.168.10.72:25060;branch=z9hG4bK6c6f4860.
From: "Max M..ller" <sip:245@192.168.10.72>;tag=as3cafd135.
To: <sip:arthur@192.168.10.72>;tag=cvovqkf6i5.
Call-ID: 4c640ebd11d192e15bfc6d3a667684ce@192.168.10.72.
CSeq: 102 INVITE.
Contact: <sip:arthur@192.168.10.221:34381;line=478vzxb3>;reg-id=1.

Proxy => Asterisk
U 192.168.10.72:5060 -> 192.168.10.72:25060
SIP/2.0 180 Ringing.
Via: SIP/2.0/UDP 192.168.10.72:25060;branch=z9hG4bK6c6f4860.
From: "Max M..ller" <sip:245@192.168.10.72>;tag=as3cafd135.
To: <sip:arthur@192.168.10.72>;tag=cvovqkf6i5.
Call-ID: 4c640ebd11d192e15bfc6d3a667684ce@192.168.10.72.
CSeq: 102 INVITE.
Contact: <sip:arthur@192.168.10.221:34381;line=478vzxb3>;reg-id=1.

When canceling the call, asterisk sends

Asterisk => Phone
U 192.168.10.72:25060 -> 192.168.10.221:34381
CANCEL sip:arthur@192.168.10.72 SIP/2.0.
Via: SIP/2.0/UDP 192.168.10.72:25060;branch=z9hG4bK6c6f4860.
Max-Forwards: 70.
From: "Max M..ller" <sip:245@192.168.10.72>;tag=as3cafd135.
To: <sip:arthur@192.168.10.72>.
Call-ID: 4c640ebd11d192e15bfc6d3a667684ce@192.168.10.72.
CSeq: 102 CANCEL.

The Phone responds:

U 192.168.10.221:34381 -> 192.168.10.72:25060
SIP/2.0 404 Not found.
Via: SIP/2.0/UDP 192.168.10.72:25060;branch=z9hG4bK6c6f4860.
From: "Max M..ller" <sip:245@192.168.10.72>;tag=as3cafd135.
To: <sip:arthur@192.168.10.72>.
Call-ID: 4c640ebd11d192e15bfc6d3a667684ce@192.168.10.72.
CSeq: 102 CANCEL.