I' ve got a new micro sim card for iphone4S. But from that, I can not send message to any number. what's the problem? Message receiving is ok.
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What plan are you currently on? Try restoring iOS via iTunes to make sure you have a clean state again. Afterwards, try putting in the Fido SIM and let it activate. Then. try seeing if you can text someone. By the way, Is this iPhone 4S a locked FIdo one or unlocked one?
If the above doesn't work, try calling in to FIdo customer support.