Question : Calling a computer

Looking into the possiblities of someone calling a computer.  I would like to be able to have some script created to where if the users presses a number then the program will do something and do something else if they press a different number.  We don't have VOIP but was thinking someone could call directly to the computer.

I believe there are third-party programs out there but was looking to build one myself.  

Thanks.

Answer : Calling a computer

Are you running windows or linux on this system?  You could certainly install a pstn card such as those made by Digium or Sangoma into the system and install Asterisk on it.  The system would then answer when somebody calls and you could play a message and do pretty much whatever you want.