I’ve got an older PBX w/ the Fax module installed (freepbx13) that suddenly stopped getting faxes, and all of the calls are exactly 27 seconds, I tried bumping the RTP time out from 30 to 90 seconds to see if that helped w/o any change.
we did have a some jitter issues last week that appear resolved for voice calling last week, and are not seeing latency or dropped packets now though.
You need to providing logging to show who is clearing the call, and, as far as possible, why.
27 seconds is an unusual time for Asterisk, itself. The most common times are just over 30 seconds when the SIP handshake fails to complete (normally wrong external addresses) and around 10 seconds, for DNS problems.