Harbor Mobile looks interesting. They don't seem to be a traditional MVNO. Their website even suggests that if you don't want to buy a SIM card, you can use an unactivated AT&T SIM card. They also support Wi-Fi calling and HD Voice (VoLTE), which the MVNOs never seem to support. They also can't port numbers from AT&T. It almost seems that they are simply pooling everyone into a large corporate AT&T plan.
I emailed them to see if an AT&T MicroCell can be used, which my parents need at their house.