Hi @Tamara
It sounds as if you need a new sim. Pop into any O2 store and they will give you a new one..
Try selecting Vodafone in network settings. After the error message reselect O2 and any stuck messages should come through.
You also need to check that none of your friends have been added to the reject list in message settings
Finally check your postcode for network problems in your area :: https://status.o2.co.uk/
Hi @Tamara, once you get a chance to try what @Cleoriff and @jonsie suggested, it'd be great if you could let us know if it sorts the issue or if you need any further help 
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@Tamara wrote:
02 said there is no problem with my SIM and their seems to be no problem with my network
O2 would have absolutely NO idea if there was a problem with your sim @Tamara
You are the one who uses it not them. They also should replace it without question.
Have you been instore to ask for a new one?
