https://www.vice.com/en_us/article/5dmbjx/how-hackers-are-breaking-into-att-tmobile-sprint-to-sim-swap-yeh

Hackers are now getting telecom employees to run software that lets the hackers directly reach into the internal systems of U.S. telecom companies to take over customer cell phone numbers, Motherboard has learned. Multiple sources in and familiar with the SIM swapping community as well as screenshots shared with Motherboard suggest at least AT&T, T-Mobile, and Sprint have been impacted.

This is an escalation in the world of SIM swapping, in which hackers take over a target's phone number so they can then access email, social media, or cryptocurrency accounts. Previously, these hackers have bribed telecom employees to perform SIM swaps or tricked workers to do so by impersonating legitimate customers over the phone or in person. Now, hackers are breaking into telecom companies, albeit crudely, to do the SIM swapping themselves.