Port forwarding" must be activated in the configuration of the WAN. For this purpose, the port range used by the VoIP telephone must be assigned to the local IP address of the VoIP telephone. With a VoIP Gigaset, for example, both (IP address and VoIP port range) can be defined / read out in the configuration of the telephone.

Since I also had problems with this: The corresponding ports are listed in the settings of the Gigaset (WebUI), under "Telephony" → "Advanced VoIP Settings" (SIP, RTP port).