Hi,
0 is ok, but you have to dial it, for example for a call to germany you should dial 00049…, first 0 will be cut off because of trunk outgoing prefix and the dial to your sip provider should look like Dial(SIP/…/…,0049…). national calls with only 0 leading zero would be dialed 00ZXXX, first 0 will be cut off again, so dial will be 0ZXX… .
If you use our client, der is a default trunk prefix configured at appliance->services-telefony that will be applied in case no other matching prefix is used.
Best regards,
Steve