Had a similar problem, found out that on the HALA prepaid all you can use your GPRS for is browse and download from their WAP portal and send MMS's... you cant do anything else.

If you are on a postpaid system then it should be working... just make sure your access points on your handset are defined correctly.

If you still having problems, let me know your handset model and ill let you know what else you can try.