There is another discussion going on in here on this same topic but, it's marked answered and everyone agrees it is NOT the correct answer.
http://support.t-mobile.com/thread/12780?tstart=0
So we'll try this again.
They page found at https://my.t-mobile.com/Billing/OneTimePaySignUp.aspx does not work for many of us.
Once on the page you are meet with a spinning circle of dots and the waiting never goes away. So basically we can NOT pay our bill simply and convenently through the standard method provided to us by T-Mobile.
It's worked for me and others in the past, that it goes back to the aforementioned spinning dots.
T-Mobile, we all need this fixed ASAP. Please get someone to look over this page. there are errors throughout it. Anyone with basic knowledge in web design can pick this page apart. Cut out the bells and whistles and please just let us pay our bills, that's what you want isn't it?