Random Silent Calls

I am having issues with some (not all) of my calls being silent, for some reason some calls attempt to connect directly to my SIP TRUNK provider (voip.co.uk) instead of relaying via Asterisk like all the other calls, my firewall is currently configured to only allow the Asterisk server to make outbound connections.

Transferring the call to another client will often fix it.
This issue occurs on both Cisco IP phones and on Jitsi.

I’m guessing, but do you mean the media goes direct to the provider, but the signalling goes via Asterisk. That is the result of incorrectly setting the directmedia option (or its obsolete synonum, canreinvite).