Possible cause 7 of 9:
Your mobile phone is locked to another service provider
If your mobile phone is locked to another service provider and you're trying to use a SingTel SIM, the display will tell that your SIM is invalid.
What you need to do
Before you can use a SingTel SIM, you need to unlock your mobile phone using an unlock code. To get the unlock code, contact the service provider that locked your mobile phone.
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