https://www.theregister.co.uk/2020/01/13/sim_swapping_study/

Four Princeton University eggheads have published a report showing that the five major US mobile carriers implement weak authentication techniques, leaving customers vulnerable to SIM-swapping attacks that transfer victims' phone numbers to devices controlled by scammers.

Such attacks have been a problem for years, but have become particularly damaging as more websites have implemented two-factor authentication procedures that rely on control of a given phone number. In September last year, Twitter CEO Jack Dorsey temporarily lost control of his Twitter account following a SIM-swap attack.