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Exactly the same way that it's interfaced to anything else through RS232!

All the modem sees is characters arriving at its RS232 port, and all it does is send characters out of its serial port; it is completely irreleveant where those characters came from, or where they go to as far as the modem is concerned - it could be a user typing manually at a keyboard, a PC, a microcontroller or anything else.
It makes no difference to the modem.

The modems AT Commands Manaul and other documentation will tell you what commands it accepts, and how it will respond.

Do remember that most modems require the use of hardware handshaking by default.

http://www.keil.com/forum/docs/thread10238.asp

"The modem neither knows nor cares what is connected to it - the interface works as defined in the modem Manual irrespective of whether you connect it to a PC, an ARM, an 8051, a dumb terminal, or anything else!"
http://www.keil.com/forum/docs/thread9113.asp

http://www.keil.com/forum/docs/thread16305.asp

etc, etc,...

Exactly the same way that it's interfaced to anything else through RS232!

All the modem sees is characters arriving at its RS232 port, and all it does is send characters out of its serial port; it is completely irreleveant where those characters came from, or where they go to as far as the modem is concerned - it could be a user typing manually at a keyboard, a PC, a microcontroller or anything else.
It makes no difference to the modem.

The modems AT Commands Manaul and other documentation will tell you what commands it accepts, and how it will respond.

Do remember that most modems require the use of hardware handshaking by default.

http://www.keil.com/forum/docs/thread10238.asp

"The modem neither knows nor cares what is connected to it - the interface works as defined in the modem Manual irrespective of whether you connect it to a PC, an ARM, an 8051, a dumb terminal, or anything else!"
http://www.keil.com/forum/docs/thread9113.asp

http://www.keil.com/forum/docs/thread16305.asp

etc, etc,...