All you'd need is a cellular modem (or DevKit) with audio (which is most of them) - either pipe it through the PC's sound card (line-in/out), or connect the headset direct to the modem (might need a small amp or "matching" circuit in there?).

Then use hyperterminal to dial outgoing calls, answer incoming calls, and send/receive texts.

As already noted, some cell phones make the necessary connections available - possibly via USB...