I was talking on the phone for about 15 mins or so and out of nowhere the phone powered off and back on it has done that three times i just purchased the phone on the 26 of this month
Well, that doesn't look normal. With regards to call, for example here with my carrier, they have a 10 minute limit per call, you will have to dial again after that. I suggest that you take your phone back to have it check. Make sure to describe the issue so it can be resolved.
Unfortunately I have been experiencing the same thing. I plan on using my warranty to get a better phone, this is hazardous seeing as though anything may warrant a call for help and BOOM your phone not just cuts off but powers down completely and takes forever to power back on.