it depends on you electronical capabilitys and how much u want to invest.
- you can use a phone like grandstream GXP series (or other cheaper) and couple a relais via a transistor to the ring indicator led.
you can switch nearly anything over the relais (lamp / horn / flashing strobe / a fogger )
simple and about 100$ (not good for resale)
or
you can use a microdevice like PIC from Microdevice with a ethernet port and ip stack and let this device login as second Device in BPX or snief the port to find out that the port is ringing and swich a relais from this device.
not so simple but not bigger then half size of a box of cigaretts.
good for resale.
about 100$ each unit but i think you have to pay some tousend $ for enginering and programming. (5.000 $ ?? not shure)