This post is for my own reference. Although my primary mobile device is Motorola Defy because my iPhone is not able to support 3G network, I sometimes have to switch my SIM card over to the iPhone, especially when my 3G network is down which means that I would not be able to make or receive any calls or text messages.

1. Put AT&T SIM card or any working SIM card (other than Celcom) and power on. Wait for signal bar / no network display.

2. Go to Settings, then Carrier. It will search for Network. (if no Carrier is shown, go back to Home and wait a while). In Carrier, wait till all 3 networks are found. Tick “automatic”

3. Press power button (put iPhone on standby NOT OFF).

4. Replace SIM card with Celcom SIM card. (The message “no SIM” will pop up so just ignore it and just press the power button to put it on standby mode again).

5. Press power button then ‘slide to unlock’. You will see the network list. Choose Celcom and return to Home.

*This tip was taken from the forum of myPDAcafe.com