Sorry that I misinterpreted the log. Unfortunately, I know almost nothing about Sangoma phones. I know that if you blind transfer an incoming call to 71, it gets parked. If you then attended transfer another incoming call to 71, that unparks the first call and the two incoming calls are connected together.
You could use pjsip logger to see the SIP transactions that underlie whatever buttons were intentionally or accidentally pressed. When a phone is ringing, what options are presented? If one of the options is ‘forward’ or ‘reject’, that may have been preprogrammed to effect a park action. Or, if you have a parking BLF key set up, it’s possible that pushing that while the phone is ringing will park the unanswered call.
If all else fails you can make some test calls in and see if any option results in answering the call and immediately parking it.