I am on my 2nd zte quartz phone. Both developed the same issue. They will begin randomly rebooting themselves. Gets progressively worse over time. I am waiting for the return label to get the 2nd one replaced. Total Wireless will not replace it with a different model but they will at least replace my phone. I am hoping the third time will be the charm.
Mine did the same thing,called Tracfone and they did a different factory reset with power button and volume control button. Back-up everything or you will loose it.Cured mine..
I read that this happens to other ZTE Quartz users as well. I highly suggest that you have it checked. Please let us know what you find about it.