It is possible to use a regular phone as an intercom, but possibly only after modification (in order to signbal the other stations' ringers).

Many years ago, I had a neighbor modify a pair of Western Electric 302s that I bought from Lafayette Radio. He modified each phone so that the other phone's bell would ring when one dialed "0" on a phone. I also needed a 67 (or so) volt battery connected between the phones to power the bells.

I just bought Philips 1.9Ghz cordless phones. 3 handsets worth. They have the intercom feature. Amazon, get three with answering machine base for under hundred bucks.

Push conference & then 1,2,or 3 and it rings just that phone. Think GE's 1.9ghz does it too so maybe its a feature of the newer DECT 1.9ghz phones.