For what it's worth- Sipphone SIP numbers begin with 747.....FWD SIP numbers begin with 393. There are other SIP networks too, each have their own three digit identifier. Some SIP network "interoperate" but not all of them do. This "peering" between networks is voluntary. FWD and Sipphone are part of the largest peered SIP network.

Another thing.... I believe you can use STUN servers belonging to non-native networks. In other words, it is possible to use FWD's stun server with Sipphone devices, and vise-versa. I don't see why anyone would do this, but it can be done. What cannot be done is pointing your IPKall PSTN number to a 747 type SIP number and have the device you hope to use authenticate to FWD. 747 SIP numbers are part of Sipphone network, use proxy01.sipphone.com with them.

I hope this information helps you.

M5