Someone is using my phone number to pay for iTunes Store purchases

I have received the bill from my mobile operator (postpaid) which includes 3 purchases from iTunes Store which I never made. Moreover, I have the only payment method authorized under my Apple ID - a credit card. There is no records on these purchases in ITunes history what makes sense - it was made by someone with different Apple ID. I wonder how it is possible - the only assumption I have is that someone linked my phone number with his or her Apple ID, but I did not confirmed it by code, never received that SMS.