When you are connecting your mobile phone or handphone to wifi or wlan, you still get charged fee for gprs when you are :
1) browsing the website.
2) downloading java games or application.
3) download android games or application.
4) playing games.

Why this happen? It is because you didn't change GPRS to WIFI / WLAN in profiles settings yet, There are many profiles settings to be set :
1) Java setting,
2) Java games setting,
3) Service setting,
4) Browser setting,
5) Android app default setting,
6) Android games setting.

Different phone has different setting location, I give u example for my phone below :
To set java games : select game > options > receiver > java sim1 profile > WLAN

To set safari browser : safari > setting > profile > WLAN

After set all things from GPRS to Wifi / WLAN, you can test it by the following steps :
1) You can go to reset your gprs counter to 0. The gprs counter is on Call History for some mobile phone.
2) Try to online browse some website / download games / play game.
3) Check gprs counter.

Note that gprs delay to charge the fee. Last time I was stupid enough, I used my mobile phone browse website, download games, play games, then I go check my phone credit balance, the balance is still the same, so I though I was using wifi, and not using gprs. But on the next day, I checked my phone credit balance again, OMG! It charged RM25 for gprs! So if you want to check gprs charge, don't check with your credit balance, you can check with gprs counter.