HI,
Here is the invite coming in from Gradwell in your trace (193.111.200.56 is a Gradwell server)...
1198751901.252474 193.111.200.56 -> 84.92.24.252 SIP/SDP Request: INVITE sip:6271749@84.92.24.252, with session description
1198751901.253965 84.92.24.252 -> 193.111.200.56 SIP Status: 404 Not Found
It's looking for an entry of 6271749 and not finding it. I think if you create a PTT-DiD entry in trunks with a start and end of 6271749 then your call should terminate correctly. Make sure to send the call to a registered phone in the inbound open route.
Kind Regards
S