Your account requires a password and a passcode. All customers have been asked to pick a hard to guess passcode, NOT the last 4 of the social security number.
Please change both and do not give the information to anyone. Your password should be alpha numeric and not a password you used before. The passcode/PIN shoud not use numbers that are easy to guess, like birthdays, anniversary.
It is unlikely someone can 'hack' your wireless account with both these set up.