Then you call your old carrier, endure their pathetic begging not to switch, give them that number and within a short time (sometimes only minutes), your phone is now on the new carrier's network, with your old phone-number intact.
This is a much better answer than forcing platforms to provide service to users whom they judge to be criminals or otherwise undesirable, but the platforms *hate* it.
23/
They *say* they hate it because it makes them complicit in crimes ("if we have to let accused fraudsters transfer their address books to rival services, we abet fraud"), but it's obvious their objection is *really* about being forced to reduce the pain of switching to a rival.
There's a superficial reasonableness to the platforms' position, but only until you think about Mark, or K Renee, or other people who've been "unpersonned" by platforms with no explanation or appeal.
24/