I want to put a phone jack in a cordless phone?

I want to put a phone jack in a cordless phone that I already have. I want to be able to use the cordless phone for a wireless dial-up if it works it works if not then the phone goes into the trash.

How does an expandable cordless phone work?

I have Vonage, so I only have one phone jack to plug in a phone. Does a cordless expansion phone have to hook into some kind of jack or do you just have to plug the second base unit into a power outlet? How does that work? How close do the 2 units have to be to each other?