Hi, we installed the free version of 3CX to do some testing. We have setup a SIP trunk and incoming and outgoing calls are working correctly. But when we call the number of the SIP trunk we don't hear any tones on the phone. The internal phone that is being called is ringing but on the phone that is calling we don't hear anything. We have contacted the SIP trunk provider and they don't know what the problem is. Does anyone know what the problem could be? Many thanks.