I have recently gone back to an earlier backed up image for other reasons, and haven't looked at telephony with this one yet, but I am almost certain I had that firewall rule in there, but it was tcp, not udp (would help if I knew the difference! *googles*). Hmm, does it make much difference? Of course I will try again with the firewall rule you mentioned. FYI currently I have this...
udp 4569 to 0 0 core_input Delete
udp 5060 to 0 0 core_input Delete
udp 2000 to 0 0 core_input Delete
tcp 2000 to 0 0 core_input Delete
tcp ipv4 5060 to 5060 0 0 core_input Delete
tcp ipv4 443 to 443 0 0 core_input Delete
tcp ipv4 22 to 22 0 0 core_input Delete
Obviously I will add that rule in and have another stab at it soon (tonight hopefully) and let you know, but I have a feeling we'll get the same result.
I would very much appreciate any investigation into this issue, and will of course help in any way I can.