I should start by saying there is not a single scenario where you should have a T-Mobile voicemail account set up without the password being enabled. The reason for this is access can easily be gained to any T-Mobile voicemail account unless you have your voicemail password enabled.
To provide an easy example if you have any VoIP soft phone that allows you to configure the CID (Caller ID) displayed when you dial other numbers then all you need to do is enter the number of the voicemail you want to access and call that number. This will provide you immediate access to that voice mailbox unless the user has their voicemail password turned on. As you can see this provides an immediate security risk and would allow anyone at any time to access your voicemails without your knowledge. So again no matter what T-Mobile device you have make sure that if your voice mail is setup that it has the voicemail password enabled.