It has to be the same geograhic area and there are numerous exceptions if you are served by a small local telephone area. You can also incur fees and cost of change over, though not all carriers do this
I assume these are mobile phone numbers, not landlines.
Yes they were mobile phones. They also did not switch phone companies just addresses. I wonder if they chose to switch companies and stay in Boston if they would have issues.
Great. We have corroboration from the site that the observations pulykamell and I had are accurate.
I’ve had the same cell number for five years. It thinks I’m with Cingular but I never have been. Is this site just making random guesses?
It thinks my number belongs to Sprint. 
Uh, no. I used to have a Sprint wireless phone, but with a different number. I received my current number from Qwest, which no longer has its own cellular service, now contracting with Verizon. I kept the number when I moved over to AT&T in 2004, and it has stayed with me through the name change to Cingular and back to AT&T.
Here is another NPA-NXX search tool I’ve used before. DETAILED Telephone Number / Prefix Locations in any Area Code (with maps) I can’t vouch for their up-to-the-minute accuracy, but I’ve never hit a counterexample there either.
As noted above, that identifies the telco which owns an NPA-NXX block, not the actual telco servicing a specific (and possibly ported) phone number.
This site does not report my land line number (which I’ve had with my current provider for over 9 years) correctly.
nm