Sounds like my issue might be elsewhere then, thanks 
If it's internal that's causing you the issue, have you got your router set up to forward the correct ports (assuming you are on a single public IP with NAT)? You need to have ports 10000-20000 forwarded to your server as well as the 5060 control port.

If it's internal that's causing you the issue, have you got your router set up to forward the correct ports (assuming you are on a single public IP with NAT)? You need to have ports 10000-20000 forwarded to your server as well as the 5060 control port.