If you want to have different phones each be a different extension you’ll need on either have a phone switch which all your phones are hooked up to, or sign up for a service which does this for you.
If you wanted to setup your own phone switch Asterisk can do this. It’s a free download, you just need to put it on a computer and get phone service through a SIP phone provider. They can then port the phone number to them and they would be your telephone provider so you would pay your bill to the instead of your local telco.
Now if you don’t want to run your own phone switch you can get a VOIP provider which will handle the phone switch functions for you. You just need to get a bunch of VIOP phones (which the provider will be happy to sell you).
There’s a company called DigiLink (I’m a customer of theirs) and they can help you setup either option.