I tried that. But my credit still keep on reducing. Any other solution??

under cydia, go to section, look for system, scroll down to S n look for SBSettings..install tat..it may ask u to respring/reboot ur device
once that is done, at ur homescreen, swipe ur finger at the carrier signal bars.. a menu wud drop down n u can see a box with E in it and a red light just below it..tap on it once, and the red light wud off.. at this stage, just respring or restart ur phone and launch safari..try to load a website..
by right, u'll se Cannot Open Page: Safari cannot open the page etc..
tats it..u wud nt use up ur credit for data now
the E beside the carrier wud still be thr but u need nt to worry abt it..
i have tried n tested this method many many times...it works

by default, sbsetting only display the 3G toggle. you will need to display the EDGE toggle. then disable both of them. your iphone should not be able to get a data IP address. so no data conn. credit won't fly off..

if you did this, and still get a IP then.... dunno liao