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Did they ever send for you?
Do any texts send for you or none at all?
Can you receive texts?
Are you sure that you're not sending MMS messages which need a data connection and cash credit to send (30c each)?
Note that some messaging clients will automatically convert "long" SMS texts to MMS.
You seem to be using an iPhone in which case are you sure that it's not iMessages rather than SMS messages that you are trying to send?
Hope this helps.