Not only that, you can only purchase a new block of 5 GB after your current one has already been used up. Before then they simply don't take your money - unless you set up autopay. And you can't purchase time online, because the URL forwards to nowhere - see above lack of any support facility. You can't even report this breakage.
So you have to do it by phone. But there are two systems; first you have to load money onto your account, then you can use the money on your account to pay for a new 5GB block - but these are two different systems with two different numbers and they each require you to punch in your phone number and a PIN code. Different PIN codes, if I recall. And again, no human support at all.
But then let me tell you about our T-Mobile cell phones here in Budapest. My phone started sending me "premium" SMS messages at 1500 ft a pop (about $7.50 per premium message) until the entire balance was used up. As service messages so they don't show in the Inbox. T-Mobile was able to block that, but my wife's phone started mysteriously "accessing the Internet" (it's not a smart phone), again at something like 500 ft per access, invisibly, until her balance was used up. The only recourse is to go to a T-Mobile store and stand in line for half an hour, because they do kind of have phone support here in Hungary, but the only thing they can tell you is to go to the store and wait in line.
The store has no permission to escalate to technicians. The people in the store "disabled" Internet on my wife's phone three separate times; the theft continued each time. Sometimes they'd refund some money and apologize and swear up and down that everything was disabled.
Finally, it seems to have stopped on its own. Maybe it was a system misconfiguration, maybe not. It's impossible to know, because clearly T-Mobile itself has no idea.
Their Internet stick (in Hungary) works great, though. So I continue to use them - it's just that if there's ever a problem they are entirely unable to fix it. The entire company is so Balkanized and so incredibly poorly managed that they can only barely function if nothing goes wrong.