I would really appreciate it if you'd clue me in. I spent roughly a half-hour trying to figure it out last night, learning only one certain fact in the process: That t-mobile's online support is full of lies.
Summary of my experience with their site last night, which you should totally feel free to ignore:
I go to my.t-mobile.com and log in.
I look for something that says something like "Billing" (on the theory that my monthly bills, which I never look at, should detail my usage). See nothing. Mouse around; find that if I hover over "Support" some subtabs appear, including one labeled "Billing" — which, I delude myself into thinking, looks promising. So I click on it.
Nothing on this page about viewing my bills either. But there is something that says
Check your usage on My T-Mobile To check your usage on My T-Mobile, log in and click USAGE.
(Yes, really: "USAGE." All caps.)
Since I had started by logging onto My T-Mobile, I was pretty sure there was no USAGE there to click on. But I looked anyway. Nada.
So, in summary, after half an hour on their website, I have not found a way to look at my bills, nor a way to view my usage. And the only fact I have ascertained is that their online help directs you to stuff on their site that doesn't exist.