[w8rbt]

SS7 attacks.

In May 2017, O2 Telefónica, a German mobile service provider, confirmed that cybercriminals had exploited SS7 vulnerabilities to bypass two-factor authentication (2FA) to make unauthorized withdrawals from users' bank accounts. The criminals first installed malware on people's computers, allowing them to steal online banking users' account credentials and phone numbers. Then the attackers purchased access to a fake telecom provider and set up redirects from the victims' phone numbers to lines controlled by them.

https://en.wikipedia.org/wiki/Signalling_System_No._7