I have the same phone and the same problem. I called the company and they said it's a software issue and there's nothing that can be fixed on it. They said I'd have to send it in and my company would have to give me a loaner phone. I talked to T-Mobile and they ended up giving me another phone that's also ZTE.. it was doing the same thing. So am I installed an App from the app Store I downloaded an automatic cache cleaned ,( cache cleaner 2018) and the problem has stopped for now. I like the phone but I hate this problem but so hopefully this helps.