how do i see or listen to my voicemail online

Hm, very interesting!  You might want to reach out to care to ensure that your Visual Voicemail feature has been added to your line of service.  This is not something that is visible on your my.t-mobile.com, but an Account Expert will be able to see it.  If it needs to be added, you will need to restart your device and give it at least 2 hours to provision on your account.