Line 4: 1. INVITE sip:192.168.1.81 SIP/2.0
shows that the Matrix is configured incorrectly, because there is no number that it is calling. On an incoming call, the user part of the SIP URI (and the To header) should normally be the DID number that is being called.
If for some reason you can’t set up the Matrix to do this, you could set up a context in Asterisk to accept the null call.
Please explain what is going on. Why did you set up a chan_sip trunk? In your thread Custom extensions , you said that all was working and now you are changing everything?