Hardware Modems
[... omissis ...]
External Modems that connect to a serial port (RS232 - 9 pin, 25 pin or maybe db8) don't need the complicated software to do the modulation/demodulation. If you have an old style serial port, an extra plug for a wall wort, and the appropriate serial cable and port(for PC's many need 25 pin to 9 pin adapter/ and cable), These work extermely well.
These modems don't need a special driver. If your modem connects to the serial port and you know the COM port, the device name is /dev/ttySx, where x is one less than the COM port number. For example, if your modem is on COM2, the device is /dev/ttyS1, or if your modem is on COM1, the device is /dev/ttyS0. This is because computers start counting at 0. You can use this device name to configure your dial-up connection.
If you only have USB ports, you may find a USB to RS232 adapter, but that will require a driver... it should probably just work, but this author hasn't tried one.