Trouble shooting modems can be categorized into two parts:
You should check that the internal modem is using the same basic sequence as any other I/O card. First, check its hardware and software configuration; then check for system conflicts, and check for correct drivers.
Trouble shooting internal modems is like troubleshooting any other internal PCI cards. Checking the hardware of the card and then proceeding to the software part of troubleshooting and finally checking for system conflicts.
If an external modem is being checked, it must be treated as an external peripheral, with the serial port being treated as a separate I/O port.
In the case of troubleshooting an external modem the device should be treated like any other external device connected to the computer.