Two good ones that I have used:
Viscount Solo and the TeleDoorBell. They can wire up between a POTS line and your phones or an unused CO port on a phone system. One of them has built in "talk power" for this and the other one has an add on module for it. There are pros & cons to both. One is better than the other for each job. They both have distinctive ring for the door, call waiting if your on the phone, and built in hold to put your caller on hold while answering the door. One will work on an existing 2 conductor door bell button wire, the other requires a 2 pair if I remember right.