The most reliable intercom for apartment use are the ones that go over the phone line. You bring a POTS line to the unit and program the numbers for the apartments, you can even get them where there is a list of tennants on a screen and they select a number. Most of them also have a form c relay output that is activated with a code over the phone. I have had good luck with Viking (
http://www.vikingelectronics.com), Talk-a-phone (
http://www.talkaphone.com), and Sentex (
http://www.sentexsystems.com)
Sentex is one of the most widly used and they have a pretty diverse product line.
