I'm sure you could whip something up in no time.

Two ways you could go about it, I think:

1. Software only. Packet sniff your network for the phone-call packets to determine when calls start and stop, or perhaps even just query your internet router for that information (there's gotta be a way to do it). Leave it running as a daemon on one of your computers.

2. Programmable hardware. A doohickey plugged into the telephone's end of the system. When an ordinary POTS telephone goes off-hook, there's a change in the voltage on the line (I forget whether it goes high or low). A simple timer circuit hooked up to a buzzer that resets on thosse voltage changes.