How can I get an intercom in my house like the ones where you dial a code on a phone to work?

I want to make one. Or Buy one. I'm talking about regular phones where you dial numbers or a code such as #90 and it goes to every speaker and you can talk. Like in a school, or a business.

How can I get an intercom in my house like the ones where you dial a code on a phone to work?
You can either buy or make a whole house intercom system, or you can buy a electronic telephone key system that has an intercom option. I know many of the Panasonic electronic telephone key systems offered intercoms as an option. You could probably pick up a used system inexpensively at an online auction site.