iPhone wont connect to 3G network

A couple of days ago i notice that my phone was only using the GPRS connection and not the 3G connection. I thought it was maybe no 3G reception where i was but yesterday i was with a friend who has the same 3G iPhone and on the same network (Optus - Australia) and his had a perfect 3G connection and mine was still on GPRS.

I have tried restarting iPhone, turning "Airplane" mode on then off, turning "3G enable" off then back on, resetting iPhone (holding down power and home buttons until phone shuts down), using iTunes to "Restore" iPhone, removing sim card and re-inserting.

Dont know what else to try???