https://www.vice.com/en_us/article/v74b4d/sim-swappers-phishing-verizon-sprint-tmobile-to-access-internal-tools

Hackers are phishing workers at major U.S. telecommunication companies to gain access to internal company tools, Motherboard has found.

Once they have access to these tools, the hackers are then able to carry out SIM swapping, where they take control of a victim's phone number in order to break into email, social media, or cryptocurrency accounts.