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I've never experienced this problem whilst sending my friends an SMS or a call, I have service, bought a new phone as a solution which didn't work and I know for a fact that my number hasn't been blocked. Please help.
An error code 21 on an SMS is usually because the number is entered wrong in the contacts list. It may be entered in internatioanl format and have the zero included (eg +61 0427 XXX XXX) when it should be without the zero (eg +61 427 XXX XXX).
Have you tried entering the number in directly when you are trying to send the sms?
Never be afraid to back yourself when trying new things, just always make sure you have 3 escape routes if things go wrong.