Have you got a mobile signal?
Try turning off iMessage.
Check the numbers you are texting are saved as normal phone numbers and not iPhone numbers.
Try a reset of your network settings.
Finally do you have the correct sim for your account type as contract and PAYG are different due to different message centre numbers which the phone reads off the SIM card.
I have no affiliation whatsoever with O2 or any subsidiary companies. Comments posted are entirely of my own opinion. This is not Customer Service so we are unable to help with account specific issues.
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