VoLTE does not work on BYOD iPhone 6 (new activations). This has been confirmed by Xfinity Mobile Engineering and Operations to me. They have been supposedly working with Verizon to fix this for several days but there has been no service alert or notification about this from Xfinity/Comcast. Very unprofessional.
The result of this is also that if VoLTE is enabled on your iPhone 6 (Settings > Cellular > Cellular Data Options > Enable LTE > Voice and Data), you may not reach some phone numbers on certain carriers (eg. T-Mobile). You will also not reach the other parties' voicemail in many cases.
The workaround is to mark the above setting to "Data Only" in effect disabling VoLTE - which would mean lower voice quality and no browsing with LTE and talking at the same time.
maybe so but it would appear there isnt much difference between the mechanics of unofficial vs official. as someone else said in a related thread, it would seem that XM is keeping an IMEI list of devices sold and VoLTE features are eventually disabled on non purchased devices