iPhone owners under T-Mobile’s magenta carrier umbrella are experiencing issues with their devices. For some, the device simply restarts randomly throughout the day, some saying even every 20 to 30 minutes. Others are experiencing a blue screen that appears for a few seconds, before restarting.
According to the discussion on Apple forums, it looks like the issue is caused by the Wi-Fi calling feature T-Mobile rolled out last year. Disabling Wi-Fi calling and VoLTE seem to fix the issue for a few people on the forums.
A Redditer contacted T-Mobile, with the carrier recommending the following solutions:]
Hard restart – hold the power and home buttons down for 10 seconds until the device restarts.
Clear out your old text messages.
Do a full factory restore in iTunes, then restore from backup.