FaceTime crossover

Hoping someone can help me here 🙂! I'm in New Zealand and call my Dad (also in NZ) all the time on FaceTime. So tonight I receive a FaceTime call apparently from my father (ie came up Dad is calling). When I answer it it was some guy driving in his car in the States. Apparently every time I call dad on FaceTime, this guys phone also rings and he understandably he was wondering why I keep calling him but never complete the call. How is this crossover happening and how do we unlink??

Cheers in advance