You don't worry about that, since you're sending packet data to the modem, not voice. To connect to a computer you need the public IP of the computer (or CName for that IP) and the computer has to be using that IP or, if the computer is on a LAN, the router has to forward the ports you're using to that computer's internal IP. You'll never get anything approaching full duplex voice on a 56k modem, even if it's actually connecting at 48k (the maximum speed it can use), unless you severly limit the audio bandwidth.