When Michael Terpin’s smartphone suddenly stopped working in June 2017, he knew it wasn’t a good sign. He called his cellular provider, AT&T, and learned that a hacker had gained control of his phone number.

The stakes were high because Terpin is a wealthy and prominent cryptocurrency investor. Terpin says the hackers gained control of his Skype account and tricked a client into sending a cryptocurrency payment to the hackers instead of to Terpin.

After the attack, Terpin asked AT&T to escalate the security protections on his phone number. According to Terpin, AT&T agreed to set up a six-digit passcode that must be entered before anyone could transfer Terpin’s phone number.