You can request a copy of your info from those databases from Transunion, LexisNexis, etc. and they'll send it to you. however since they are not used in credit decisions they are not governed by the FCRA and you can not dispute or amend anything. Every phone number I've ever had is in those databases, as well as every address I've ever lived at - plus a few dozen variations that are typos or incomplete which I guess says something about my handwriting, every paycheck I've received over the past twenty-some years is in there, both regular payments and bonus, with taxes broken out and everything (a lot of businesses outsource employment verification to a Transunion subsidiary called The Work Number and in return they get all that information). There is my list of "known associates" which is pretty much all of my family living and dead since the 80s, all of my wife's family, a couple former roommates and their families. Apparently I own a sporting goods store in Austin, Tx (thats false, but can't dispute or amend). Every legal action I've been a party to is there. Every car I've ever driven is in there. I think most people would be shocked if they knew how much data these companies keep. I became aware when a police officer called me about some tenants at a rental property on a phone number I'd never given to anyone. I think the Transunion TXLop database is where I finally found the number. Since then its been my hobby to see how long I can keep my phone numbers out of that database.

I think he means that the hijackers call into T-Mobile customer support who then allow his number to be ported to their sim.

My T-Mobile number was simjacked last year, though afterwards once I reclaimed my number they let me set up a "secret word" that the person calling in has to give them and I haven't had any problems since.