Monitor Phone calls Duration

Question

Hay Every one.

Nice to be here. This is my first question so please forgive me if I say something stupid or obvious.

I'm in a company with 2 phone lines and 3 offices. Each office has both lines on 2 different phone. My boss asked me if i can make a program that monitors the duration of all calls made and from which office?  @_@

I asked some people and some say it can be done with something called a BUNGLE (what ever that is) connected to the USB. Others say connect the phone strait to the PC via modem.

Storing the time is no problem. But how does the program know when the phone line in use or which office is using it seem impossible to me.

Any one has any idea of what code or device is use. I would be very grateful.