This is a huge reason why you need to be careful about your digital communications. Everything you do across networks gets backed up and stored somewhere. This creates plenty of targets for Threat Actors to go after. This is NOT just an AT&T issue. They just happen to be the target in these attacks. All communications providers are constantly targeted.
https://www.securityweek.com/att-data-breach-nearly-all-wireless-customers-exposed-in-massive-hack/