How can you stop a hacker from getting your voicemail password to your cellphone?

No matter how much I change it my friend keeps getting it by putting a code when he listen to my voice mail. that is how he got it. I tried calling tmobile and they are telling me that no one has called to change it. which is true you do not need to call the operator to get the passcode.How can you stop a hacker from getting your voicemail password to your cellphone?If you dial #PWD# send on the phone, that resets your voicemail password to the last four digits of your mobile number, and you can go in and access the voicemail that way. That would be the only way that he is getting into your voicemail. The T-Mobile voicemail system does not allow to hear your password in the system, but he can change it on the phone.How can you stop a hacker from getting your voicemail password to your cellphone?just call t-mobile and keep complaining tell them you'll leave and go somewhere else and then tell your friend that he should stop or you will tell someone