Sprint phone bill charges.?

I was charged an extra $12 on my phone bill for 59 "international texts - Russia". I contacted a Sprint representative, asked if this could be a mistake, and they said that it was not a mistake. I asked for an explanation, and he said he could not give me one. I checked my text log, and I have texts that show up from a friend's phone number (as 720-xxx-xxxx) and are under category "texts". But, for the same phone number for a few days out of the month, the texts show up under the category "international texts" for a charge of $0.20 per text. I contacted my friend, and she was definitely not in Russia for any period last month. Can someone please tell me where these charges came from? I will have to call Sprint tomorrow, but I would just like to know before I am put on hold with them for an hour.