free phone service?

I was talking with a friend's little brother, 12 years old, and it turns out that he and his buddy (who is some kind of genius) have rigged 2 cell phones to work as 2-way radios - they can't call any other phones, but these two phones can "call" each other for free.
My question is, what else can we do for free? Do we citizens HAVE to sign up with some major phone company, or are there other ways to get phone service?
I realize there's the little matter of the fact that only phone companies own the satellites that CELL phones rely upon... but who owns the telephone lines? or who pays for their construction?

There is nothing "free" in this world....

regards
seekerof