How can someone use my daughters phone number to call me?

The other day I received 3 different calls all from my daughters phone number. At first I couldn't hear anything but then the 2nd and 3rd calls I heard kids in the background. My first thought was the kids were playing on her phone. Then I got a facetime from her. When I answered the facetime call I heard...."Oh my gosh it works." and they hung up again. So I facetimed my daughters number back and this girl who I have never seen answered. She was probably in 8th or 9th grade and there were kids laughing at her in the background. I could tell they were at school. She wouldn't say much to me and then just hung up on me. After that when I tried to call my daughter there was no answer. So I called AT&T and they said the kids have a app on their phone to do this kind of thing. So I hung up from AT&T and sent her a text message telling her to stop. Then I get a text from her mom and they live downstate from me!. Does anyone know how she did this? I want to make sure that my daughter knows what to look for on her kids phones. Scared the bejeebies out of me.