Just wanted to chime in that I had exactly the same issue with my old BYOD phone, but in my use case it can be worked around.
The phone in question is an unlocked Sony Xperia Z1 Compact, running Android Lollipop. It is not compatible with WiFi calling or VoLTE, which I think is the problem. I have two DIGITS numbers, one of which (the "primary line") matches the SIM card in the phone. My phone refuses to use WiFi for calls from the primary line, including when there is no service or the phone is on airplane mode. Setting "Use data only" in app settings does not fix this. Other non-primary lines will still use Wifi when cell service is not available.
Sign out of the DIGITS app, turn on airplane mode, get on WiFi, and sign back in. DIGITS will switch to " Device Line: No SIM Card" mode. You can look in App Settings to verify this. All calls made from within the DIGITS app, including from the primary line, will now be made using WiFi. Unfortunately, you'll lose the ability to make calls from the non-primary numbers using the regular voice network. That doesn't bother me much, but your use case might be different. I'm confident T-mobile willl fix this soon (Please and thank you!) but until then that's my solution.