> "I used a prepaid card to buy their $16 per month plan and then after that was done it let me steal numbers just by filling out LOA info with fake info," Lucky225 added, referring to a Letter of Authorization, a document saying that the signer has authority to switch telephone numbers.

From 2021:

https://www.vice.com/en/article/y3g8wb/hacker-got-my-texts-1...

EDIT: yea it's not 'registering' the number but rather rerouting a copy of all messages to another number.