iMessage activation message. Why I have to pay more than once?

Hi there. Here is my issue with iMessage:

I'm from Spain (sorry for my bad English). I buyed an iPhone 4S one month ago. I activated iMessage then, and yesterday I was checking my phone invoice when I noticed I had sent 3 international sms (to U.K.). Each one was 0'60 € I checked the number, and It is an activation number used by Apple to verify your number. I can understand this, but i can't understand why you have to pay for this verification everytime you switch on iMessage. I checked the dates when these messages were send and the other 2 messages were send when I restore my iPhone (iOS 5.0.1 upgrade and another one because i changed my AppleID). So, I think this is a bug from iOS5 that I should't pay and It's something Apple must fix.

I have read in other Spanish Forums about people with more than 10 activation messages (about 6 €) and other people without these messages, so I don't understand what happens.

Have anyone the same problem?? Any solutions? Thanks