I have a sim only deal which started in July. When my friends reply to my texts, the replies go to someone else called Maddie who is getting fed up by all accounts. I have checked the phone number they are replying to and it is mine. My sim is part of a family bundle. Someone else is getting texts meant for me. I spent hours trying to phone virgin today only to be offered multiple choices which ended up with them texting me a we link of no use. No option for me to speak to anyone. Very annoyed.
I used to work at the Genius Bar for Apple and have seen this issue before. What I think might be happening is that the person who is receiving the messages intended for you used to have your mobile number, she will have a new mobile number but the number that is now yours may still be attached to her Apple ID, she would need to go to appleid.apple.com, sign in with her Apple ID and remove the number that is now yours.
This would be my first guess at what is happening and of course this could only be the case if this other person uses and iPhone & has an Apple ID.