What mobile do you have? Knowing would really help.
There are two easy ways to use a mobile. The first is to find a junk one that supports a proper serial interface through a connector on the body (e.g. older Nokia or Siemens models). The second is to buy a small module which allows you to put a SIM card inside and send it serial commands.
Interfacing with any sort of modern phone will probably be fairly impossible. They all have non-standard USB interfaces and those that do support modem functions normally do it through custom drivers that run on top of the USB data stream.