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For sending SMS to oversea, there should have a plus + in front of the phone number.
Example: +65nn1234567 "nn" is the city code, if any.
The plus + follow by country code, city code, receiver phone number.
If not, sending SMS will failed.
Instead of using plus + in front;
you can use 008nnnnnnnnn (Starhub) & 001nnnnnnn (Singtel).
For 018 & 019 (Starhub & Singtel) this will failed.