If you are an Apple iPhone user, and from Singtel, remember not to press “OK” when you see this message: “iMessage: Your carrier may charge for SMS messages used to activate iMessage.”.
This message appears when you swop SIM card, turn off the phone, or in various other situations.
Singtel treats this as “International SMS”, and can charge a fee for it. I got hit with a $5 fee as I was frequently swopping SIM cards (to check for any SMS received on my local card). The charges appear as “AutoRoam (PAYR) SMS”.
The solution is to press “Cancel” whenever the message pops up. If you are already get the charges by Singtel, you can try calling in/online chat with them to waive the fee. I managed to get a “one-time waiver” which will supposedly occur within two billing cycles.