5 or 6 years ago, my phone number got ported by someone else without my knowing. My phone suddenly didn't work anymore. I called into AT&T right the way to ask what's going on and they said someone has "took over billing" from my account and AT&T transferred the number over. WTF? I was adamant to get the number back since that's the number I give it out to people. They won't bungle saying it's out of their hand. Finally they said they could place the number into the free pool for re-allocation which would freeze it for 3 months before it could be used again. I was concerned it could be used as a vector against my bank accounts. It was a nightmare.
So basically as long as a hacker wouldn't mind paying your phone bill for a month they could easily take over billing of it, then clean your accounts out, then cancel the number.
This is another one of those messy human areas where computer systems are either too strict or not strict enough.
