The case of the missing cell phone!

My oldest daughter has asked me for a cell phone for a few years now. I had always promised her that she could have one when she turned 12. Well last year around this time my grandma's cell phone was missing. We all looked for it, including Caitlyn. We came to the conclusion it was stolen out of her car or she really misplaced it, until my uncle received the bill. Here Caitlyn had it all along, had been using it regularly and even charging it! She was busted and had a LONG list of punishments, including being grounded for her entire winter break, and banned from getting a cell phone for her 12th birthday!
Well yr went by and she asked me about it again before her b-day. I said no, even though I had decided I would get it for her. She really learned her lesson. So I gave it to her and she was BEYOND excited! It was the best gift ever. About a month ago after only having it for a month, her new friend asked to trade phones for 2 classes. I found this out after a few weeks of "my phone is in my locker, I can't open my locker, I forgot, " etc. There Are many things to this story, lets even look past the fact that she was not allowed to have her phone in class at all.
So I have asked Caitlyn everyday the phone status for the last month. The girl had a flurry of excuses also. "It's in my locker, I can't open it, it really wasn't in my locker, it's at home, it's at my dad's, he works a lot and i can't get it, EVEN we stopped by to drop it off to you but no one was home." ALL OBVIOUS LIES! I have been so patient with this girl, I told Caitlyn several times I was going to the house and talk to the parents, she would tell the girl and the girl would promise the phone the next day.
WELL I was finally fed up. My feelings are so hurt and I had grounded Caitlyn for a month because of this. It was time to call the parents. I called the number and asked for the mom. The guy on the phone said the mom didn't live there, that he was the grandpa and she lived with him. I told him the story, he asked the girl and she said it was in her locker. I told him we'd heard that before along with tons of other excuses. He assured me she would return it the next day and if not to call him back. Yesterday came, no cell phone. Caitlyn said the girl said it was at her dad's. Last night I got a phone call from "Amanda's Mom" telling me that "the dad" had the cell phone and he works a lot so we couldn't get it back until Friday. This was very obviously Amanda, so I said, ok but I would still be calling the grandpa. I was so furious I couldn't even see straight! I was ready to go over there or call the police. I calmed down and assumed the grandpa wasn't there so I called later. I called him a few times and when I finally got a hold of him he said Amanda told him she had returned the phone that it was in her locker. I told him that not only was that untrue, but that I had gotten a call from his number at 6:30 and the person said it was Amanda's mom and I told him what was said. He said the mom wasn't there-and I said then it was Amanda pretending to be her. He said he didn't know what to do that she had lied to him. I asked him if he wanted to get to the bottom of it and I would call him tomorrow. He said ok. But I am very concerned that he will brush me off or avoid this issue. If he can't resolve it should I go to the police?? I have no idea!