https://www.vice.com/en_us/article/kz438w/verizon-makes-sim-swapping-hard-why-doesnt-atandt-sprint-and-t-mobile

Last week, we revealed the existence of a little known feature that could make T-Mobile customers a bit more secure against the rising fraud known as SIM swapping.

The feature is called NOPORT, and it makes it a bit harder for criminals to steal the phone numbers of T-Mobile customers. This attack, also known as SIM hijacking or Port Out scam, allows criminals to take control of a victim’s phone number and then use that to reset their email password, banking account, and other important accounts that may be tied to that number.