T-Mo is known to have many holes in its billing system.

T-Zones is a data service that comes with a contract account (dunno about PAYG). It's supposed to only connect to their store (ringtones, wallpapers, games), but any phone can select "Web address" and see a humongous number of WAP sites.

I buy an unlimited data service called T-Mobileweb for $5.99 which is a step up from T-Zones. It's supposed to be WAP-only, but I configured my iPAQ to be a GPRS internet terminal. It's slow and cheap.

When I roam in Europe on T-Mo (99¢/min), the T-Mobileweb keeps working without charging me anything, provided that my phone is registered on a T-Mo cell site (which I usually have to do manually when changing countries). As soon as I get that "Welcome to O2" (or whatever), I'm off to the configuration menu to avoid surprises.