2 Phones... 1 Number

My wife got this deal on her phone plan. Basically they let her add a home phone for $10. We up until then, did not have a phone, we just lived off of cellphones. But anyway, sounds good right? $10 my teenager can talk to whoever, not tie up our cellphones and everybody is happy.

Except for the number they gave us was already in use. And I don't know how two different phones can even be one the same line. We got calls for the other guy, the other gut got calls for us. It was a mess. We eventuality got the whole thing straitened out, we got us a new phone number, but still, no one could tell us why it happened, and what's stopping this kind of thing from happening again? I don't know...

You'd think that somewhere there would be a list or something... of phone numbers that are/are not in use, but dang if we could find them.