Fongo powers free calling solutions for a number of brands. This forum includes information and tips on all of those offerings. Note that your specific services may vary based on the specific Fongo product that you're using.
Port 5060 UDP is the normal port that is used for SIP signaling. Some providers offer other ports eg. 5080 UDP and Vbuzzer uses port 80 UDP .
This is the external port.
Normally you can forward a port of your choice if it is not already used for something else eg. 5021 UDP from the router to the ATA (and enter this as the local port the standard port in the ATA setup or it may be assumed by default). The ATA (or SIP client) should look after sending the outgoing signals to the 5060 port and the reply return via the internal port to the ATA via your router NAT.