Раздел Dynamic IP address, пункт 2:
For inbound calls to one of Telephone Numbers on your GoTrunk account to work FusionPBX needs to Register with GoTrunk service (and periodically refresh registration in case IP address changes). Calls will be sent to IP address which was sent in the most recent FusionPBX Registration. Since the calls will be coming from known peer (IP address of SIP Trunking service q.x.y.z in our example above) FusionPBX will accept them without requiring any further authentication.