I am not sure, if it is that easy!
I tried my (voice) prepaid sim and put in
In the Access point field, type epc.tmobile.com.
In the Username field, type GPRS.
In the Password field, type GPRS.
In the Additional intialization commands field, type *99***1#.
and got a (fast) connection.
BUT:
It does not matter, which URL I put in, I always get directed to
https://my.t-mobile.com/Login/?rc=&d...2fDefault.aspx
Here, I may connect to (my) account informations, but that's it!
There is no "options" or something like that to activate data service...