Using AT command you can easily turn your mobile phone into SMS gateway for example. We wrote about this already.
But the interesting thing that you can extend this approach to the voice calls also. Here is an experiment:
Mobile phone Sony Ericsson K320i
USB Data cable
Software: any application capable read/write serial port.
The following command AT*ECAM=1 switch on call monitoring. So for any incoming call you will get ECAV messages describes a type for the call (e.g. voice call) as well as a phone number for the second party. So actually you can develop some telecom services using your mobile phone as a telecom platform