I noticed the question you asked prior to this one was about walkie-talkies. Are you maybe asking how to make your own small phone network with just a few phones? Or maybe how to use cordless phones as walkie-talkies? Like this instructable:https://www.instructables.com/id/Build-your-own-intercom-or-walkie-talkie-out-of-tw/ And this answer:https://www.instructables.com/answers/Pls-let-me-know-the-cheapest-way-to-make-an-interc/
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Personally, I would go to a garage sale or thrift shop and just buy one for a couple of bucks. It would be cheaper than buying the components separately, and easier than assembling said components. But then, I like doing things the easy way.
+1. Among other things, it is not acceptable to connect unapproved electronics to the phone line, for safety reasons (yours AND theirs), and the required isolator box for experimentation costs about as much as a good phone. Unless you plan on developing something you can't buy cheaply, it generally isn't worth bothering. (The rules _MAY_ be relaxed for FIOS service, where there's already electical isolation between your system and the phone company's. Of course if you damage the FIOS transciever they'll bill you for replacing it, which I'd bet is several hundred bucks.) Generally, phones are not usually the right place for a beginner to start experimenting.