Account hacking may indicate someone has stolen your identity, accessed your email, knows your social security number.
You not only need to secure your ATT account with username, password and passcode/PIN , remove all authorized users. You also need to secure the email account associated with your account. My email has 2 step verification. This means no one can access it without a code sent to my cell phone.
Run your credit history, change all passwords, make sure you have deleted all old devices from your iCloud, Google or Windows accounts. Buy a document shredder, etc.
https://www.consumer.gov/articles/1015-avoiding-identity-theft