I remembered why I needed to use different ports in one case: Two of the three accounts are at the same ITSP, in this case since the IP address is the same on the two accounts you need to use different ports.
Ah, I have sort of the same case - 4 accounts with one ITSP providing 4 different incoming DID's (replacing an old ISDN connection), and 2 separate single ITSP accounts for international calls. But I am still very confused about the need for different ports. This is not a complaint, just an observation, but I do think the multiple port thing is one of the more confusing aspects of FS, and something other SIP switches seem to do without. On outgoing connections, we register with the ITSP gateway (by the way, does the SIP registration register both an ip address and a port number, or just the ip address?), and on incoming connections they either have a destination number/DID that routes them to the right place in the dialplan, or they register with us (using username/password) in the case of SIP clients. Security can be handled with ACL's. So I can't get my head around to why separate ports are needed - maybe someone can point out what I am missing? Julf