International sms sending in background without permission

It’s happening to me from last few when I receive my provider bill and find out that there are lots of international sms sent through my iphone which I never sent mostly those are sent to UK and Philippines, I am annoyed and I asked service provider and they ask to check apple. I checked both but no one was able to give me some specific reason. It’s like stealing or cheating if my phone has no record that I have sent any international sms and it’s happening monthly basis and few amounts added up to my monthly bill. They say it’s about IMessages but I don’t think so.

Kindly if someone can help me to solve this issue or how it is happening

Thanks

Ali