In Tionesta, PA I have good 4G signal but voice calls fail, by fail I mean that if a call is initiated the dialer just doesn't connect and the call is terminated, in other areas I don't have issues making or receiving calls. I'm on the $30 prepaid plan using an unlocked Blu R1 HD (Android 6.0) with the most recent updates so Band 12 and VoLTE are enabled, and "Enhanced 4G LTE Mode" is enabled in settings. In the problematic area there is a notification "Data Connection Only" and the network name doesn't list "T-Mobile" instead it says "Emergency Calls Only".
I suspect that VoLTE isn't working correctly on my phone and that this area is only serviced by LTE Band 12 with no 2G or 3G to fall back on. According to LTE Discovery the Band I'm connected to is Band 12. The data connection works fine and my connection speed is ample 20.1 Mb UL / 5.8 Mb DL according to Speedtest. I haven't changed any phone settings and phone is not rooted. The T-Mobile US LTE APN settings look as suggested as far as I can tell, the only questionable settings are MVNO type = GID and MVNO value = 544d. Also there is a second APN T-Mobile US IMS that I'm unsure of the correct values. Is there something I'm missing that could cause an LTE only connection without voice or any suggestions of how to resolve the issue. I'm currently testing T-Mobile with the objective of subscribing to the One plan if the network is ample enough in my area so The Blu R1 HD wouldn't be my longterm phone.