Doorbell connected to telephone system

Does anyone have a doorbell(or intercom) that is connected to the telephone system? We have an old (16 year old) Lucent Technology telephone system which is connected to the front door bell so when the door bell rings, you can pick up any phone in the house and find out who is at the front door. A few weeks ago, the system broke down and because it is old, the parts to repair it are no longer being manufactured. Has anyone recently installed or upgraded a similar system to this in their brownstone? Who did the work? What telephone system was installed? Any info would be appreciated. Thanks!