There are really three T-Mobile phone data products, and people get them confused. (The following is theoretical. There are numerous holes in the T-Mo system that allow subscribers to get more than they're paying for.)

T-Zones is free, but it only allows phones to connect to a commercial server that offers downloadable ringtones, wallpaper, that sort of stuff.

T-MobileWeb costs $5.99, and is supposed to provide unlimited access, but only to WAP sites (whose content is supposed to be bandwidth-sensitive).

T-Mobile Internet: three plans, from $19.99 to $49.99 (depending on what client terminal equipment is used) for complete access.