Sprint has updated its VoLTE Page as a result of the merger: https://www.sprint.com/en/shop/services/volte.html
Support for additional devices has been listed.
A couple of things I noticed:
“By enabling VoLTE, you will get access to expanded coverage on the T-Mobile network with capable devices.”
and this change to Wi-Fi Calling:
How does VoLTE work with Wi-Fi Calling?
Wi-Fi Calls will transfer to VoLTE when Wi-Fi coverage becomes poor and VoLTE is available. For this automated transfer to take place, VoLTE must be turned “ON”.
Starting in April 2020, Sprint is updating most VoLTE-capable devices to prefer Cellular networks over Wi-Fi Networks. This means that if your device has Wi-Fi Calling enabled, it will only be used if the cellular coverage in your area is poor. If you are on an active call using VoLTE and the coverage does decline, the call will attempt to transfer from VoLTE to Wi-Fi when the device is connected to a strong Wi-Fi signal.