I'm running the 11.7 A/B with GAPPS built in on an Umidigi F1. Everything seems to be running well and smooth, with the exception of VoLTE and VoWifi. I'm on T-mobile in the USA and out of the box the phone works for both on the stock software. I dialed *#*#4636#*#* and both appear to be provisioned in the dialer settings. In addition, if I go to Network & Internet > Mobile Network > Advanced the wifi calling option is available and turned on. Even so, if I make a call in airplane mode with wifi turned back on and connected, it fails... and whenever I place a call on LTE, it drops back to H+. Any help or ideas on how to get it working would be greatly appreciated. Thanks.