For any external modem, IIRC...
AppleTalk control panel: "connect via" selection should not be set to compete for that serial port.
Chooser: no printer should be configured to be competing for that serial port.
Extensions: go thru the list for any likely serial and modem related suspects that might be inadvertently disabled. Unless you have exotic additions that cause extension conflicts, all enabled would be good.
System Folder:Extensions:Modem Scripts...your modem needs to have its configuration string script in the folder here.
Modem Control Panel: port and modem type need to be selected.
Cable: the pin connections need to support hardware handshaking and have the wiring for connections between a DTE (computer) and DCE (modem) so that outputs do not end up driving each other.
AppleTalk control panel: "connect via" selection should not be set to compete for that serial port.
Chooser: no printer should be configured to be competing for that serial port.
Extensions: go thru the list for any likely serial and modem related suspects that might be inadvertently disabled. Unless you have exotic additions that cause extension conflicts, all enabled would be good.
System Folder:Extensions:Modem Scripts...your modem needs to have its configuration string script in the folder here.
Modem Control Panel: port and modem type need to be selected.
Cable: the pin connections need to support hardware handshaking and have the wiring for connections between a DTE (computer) and DCE (modem) so that outputs do not end up driving each other.