IDD (International Direct Dialing):
Any international calls, SMS, and MMS made or received while in Singapore will incur IDD charges. You can refer to the IDD rate here for more details.
Global Fees:
For iPhone users, activating iMessage for a phone number requires the device to send a silent SMS containing a verification code.
When the pop-up appears, the device uses an SMS to activate iMessage with your phone number. Once activated, you can be contacted via iMessage or FaceTime using your phone number.
To avoid incurring global fees, you have the option to opt out of using iMessage by selecting "Cancel" when the pop-up appears on your phone.