Try dialling in manually first! That may display some more detailed information about the problem, or simply reveal that there is a problem with the automatic dial up script. Make sure you are using the correct script. MacPPP2.5 will not dial in automatically; be sure to get MacPPP2.1
This could interfere with your connection. Turn this off by changing your phone number to start with
*70,
This will disable the call waiting. For example
*70,5551212
Fax software may interfere with your connection. Make sure that you are not listening for/automatically receiving a fax while you are connected to the internet.
If you have an old UART, then your modem should be set to 19200. Otherwise you will get many com errors because your modem is trying to send/receive data faster than what your serial card can handle.
Check the High Speed Modem FAQ for more information about modems. Also be sure to read Modems, modems, modems for all your modem questions.