Calls crashing while using other apps

I had just got a ZTE majesty pro from Walmart and when I try and make calls it lasts for a little bit, but when I open other apps while on a call, it crashes, ends the call, and I cannot call that person back but I can call other numbers. My fiance said she tried to call me too but none of our calls or texts would go through to one another for like an hour. What can I do to prevent this from happening? I've tried factory reset and clearing cache and data.