Does anyone know of a voip product that exists that you can use voip to simulate an analog ring down type of circuit?

My customer has been paying for a ring down type of circuit through the phone company. At building 1, they pick up the line and hear the intercom tone of the phone system at building 2. At Building 2, they can pick up the line and hear the intercom tone of building 1.

The circuit is hooked into the analog single line port of both systems. This works well for the occasional use of the circuit for page pickup and direct intercom calling. They can just as easily call the building talk to the receptionist and be transfered to whomever they need.

They would like to stop paying the phone company. They do not want to buy new phone systems that have voip circuits. A voip solution within the phone brand for one or two circuits is too prohibitive. We can't get an aerial or burial to the buildings due to logistics. Can't do a wireless/microwave solution.

It would be nice to have a solution like a Linksys/Vonage router box with two rj11's on them at both buildings. Or can something like this be modified to talk to each other?

I can even use a co. to co. or co. to sl. setup as well.

Any thoughts?

Tip and ring, the work of the devil!