The company admitted on Twitter that it stores at least part of their customer’s passwords in plaintext. This is a big no-no in this day and age because if anyone breaches T-Mobile (and companies are breached all the time), they could likely guess or brute-force every user’s password. If the passwords were fully encrypted or hashed, it wouldn’t be that easy.
But having a portion of the credential in plaintext reduces the difficulty of decoding the hashed part and obtaining the whole password.
Source: vice.com