Phones losing Registration

My phone only seem to be able to hold their registration for a few hours. My setup is as follows

Asterisk server running on a VM hosted elsewhere,

Pfsense operating as the firewall in front of a dozen or so phones.

Everything will work fine for a few hours then phones will start to loose their registration. If I clear the firewall states on the PFsense machine they will return to normal.

Clearly a firewall problem.

Normally one would re-register rather more often than that, so any dynamic rules ought to be reset.

pfSense actually has a page[1] which details the settings which can impact VoIP functionality for equipment behind a pfSense router and suggestions.