It is non-trivial, but not impossible.
You need to be prepared to do some serious background reading & research - but it can be worth it!

Your first task, obviously, is to find the full technical documentation for your modem - and read it!

With modems it is particularly true that the effort to drive things when everything goes right amounts to only 10% or so of the effort to handle all the things that could possibly go wrong! :-(

eg, it is easy to make a modem dial, connect, send some text, then disconnect;
But what happens if there's no dialtone?
or no answer?
or the called number is busy?
or the call drops before the transmission completes?
or you get an unexpected incoming call?
or ... etc, etc, etc, ...