We have one incoming SIP trunk that handles all our calls. The SIP extension 99 is set to 'Many Extension Ringing' with no other inbound call rules.

All is working well (after I figured out SIP ALG had to be disabled on our router!) except for one tiny (big!) issue.

When an inbound call is recieved from an international number (say, with a CLI of +3531234567) as soon as the call is answered by any of the handsets in the MER group the call immediately drops.

No other calls seem to drop in this way (although there have been one or two reported).

For the dropped calls, the log says call duration 0 seconds, and the inbound caller says that the call 'rung out'. I am unable to test and diagnose this as I don't have an international number to dial from!

Are there any rules/configuration that I have missed that may cause this issue?

FYI, the firewall is set to 'Low Security' (if this matters!)

Thanks in advance!