Configuring Wi-Fi calling with MVNO Given a T-mobile issued iPhone that is Wi-Fi compatible and SIM unlocked,

How can I configure wifi calling to work with a corresponding Mobile Virtual  Network Operator (MVNO) (i.e. Mint Mobile)?
You do that by inserting the SIM-card from the MVNO, and then access Settings on the iPhone. In Settings, choose Cellular and then Wi-Fi Calling and turn it on.