An internal modem is basically an external modem and serial port mounted upon a PC bus card. These are cheaper than external modems as they do not require a power supply or a chassis.
Internal modems work fine for remote serial console applications. They are especially attractive for computers at co-location sites, as those sites charge according to space and power consumption.
Check that your internal modem preserves its setting across a power cycle.
Ensure that the interrupt line and port address space used by the internal modem's serial port do not conflict with that used by any other pre-existing serial ports. Alternatively, ensure that the internal serial port can be disabled, freeing its interrupt line and port address space for use by the internal modem.
Be careful not to confuse an internal modem with a WinModem. An internal modem does not need a special device driver, but appears to Linux as a stardard serial port.