I wouldn't have thought it would be this easy to steal someone's identity and their mobile phone number at the same time, but I stand corrected.
It's not supposed to happen, but evidently it does, and it seems that folks whose phones get their service through carriers who operate or sell their services through retail storefronts (AT&T, Sprint, T-Mobile, Verizon, The Apple Store...etc.) are most at risk.
It makes a good case for using a lesser-known service provider (such as TracPhone Wireless) that uses the big boys' infrastructure and coverage, often at lower cost.
It also makes a good case for keeping a land line, which isn't as easily hijacked...although the use of land lines seems to be growing increasingly out of fashion.
Anyhow, the article linked above includes information on how to reduce the risk of your mobile account being hijacked. Even that isn't foolproof, but it's better than nothing. Forewarned is forearmed.