If you are buying a used modem, do not buy anything slower than 28,400 bps. Modems come in various shapes and sizes. The disadvantage to this modem is that if the modem hangs you must turn off your computer to reset it. The unit connects to your cable tv box instead of your phone line. Not available in the area at this time.
External modems can work on almost any computer that has a serial port.
I get a lot of questions about this. Here are some common modem lights and what they mean. Your modem manual should include a complete list. The modem may answer the phone. The modem is connected to another computer.