I just transferred my number off VZW; they did everything to prevent me from generating the transfer PIN that they require. That generator has been moved around the customer portal, it's now at the bottom of a seemingly-unrelated page and requires scrolling to find., so my MVNO's navigation instructions were no help
When found it is easy to run out of attempts and be given a 24 hour hiatus before further attempts. None of the 2FA texts arrived, use the email option if you need to do this. Texts suddenly worked perfectly minutes later when I called CS and they wanted me to use their digital assistant instead, was practically bombarded with imprecations to talk with their AI.
Perhaps incidentally, after I made it known that I wanted to transfer, my calls to CS were routed to a long queue. yet when I put in another SIM and called their CS I was answered promptly until they associated that number with me. Eventually, after days of trying, I got a transfer PIN that worked, unlike the one given to me verbally by VZW CS that turned out to be "invalid".
Point being that things might be in place to prevent transfer, but they also might be used to prevent valid transfer if it so benefits the carrier.
If I had one of those currency accounts I'd use an MVNO SIM and keep that phone number generally out of circulation and not associated with me anywhere official. There are plenty of MVNO's, easy to have several and rotate every so often. If thieves went after my regular number it would have no effect on my accounts. Those prepay SIMs are cheap to buy and keep.
But I'm not a technology consultant, except to friends and family.