Is there a firewall protecting the new fop2 server, and if so does it have a rule allowing connections on tcp/4445 from the client machines to the server?

I have an odd issue on a customer site when their users dial using fop2.

1. User enters target number in fop2 and hits dial.
2. users extension rings and they pick it up.
3. user only hears their own voice echoing back, not the dial tone or ringing tone of the call progressing.

This issue is intermittent, and if the user retries a few seconds later it normally goes through.

Fop2 is running on a separate server from asterisk, and has been fine up until a few weeks ago. Nothing hugely useful in the asterisk logs for these calls. They show call connecting then immediately disconnecting.

This issue is maybe affecting one call in 200 and appears to happen to all users. Asterisk is in device and user mode, with asterisk 11 and FreePBX front end.