Are you sure that old fashioned "Call Forwarding" isn't an option on your old phone system? Sometimes setting up call forwarding is as easy as dialing *72 and then dialing the number to forward all calls to. Later you can dial *73 to turn it off.
If traditional Call Forwarding from the central office or PBX really isn't an option for you, here's an approach to doing what you asked:
If the phone in your office is a plain old analog telephone, then the hardware you need is called an "FXO" interface for your PC.
If you look hard enough, you can find single-port FXO solutions for US$30-$50.
Most modems are only designed for data, so they don't act as an audio I/O device for your PC. An FXO can detect rings and go off-hook (or go off-hook and then dial) and then start acting as an audio I/O device between the POTS line and your PC, and then go back on-hook at the end of the call.
There's a highly customizable open source VoIP gateway (and much more) software package called Asterisk that you could probably use to gateway the incoming call to whatever device or service you want it to go to.
Caveat lector: I haven't done all this myself, so this is just a possible lead for you to look into. Maybe someone with more DIY VoIP experience can tell me if I'm off my rocker.