my cellular provider (T-Mobile) uses UMA/GAN to switch between wifi enabled cellular network access and cellular network access proper on the fly

Republic specifically does this; it's their whole marketing gimmick. You pay a low amount for a modest data cap, but the phone seamlessly prioritizes WiFi networks when available.

I've been using them for years and it works fine.

Not sure about republic, but many phones implement WiFi calling.

His network probably allows Wi-Fi calling and text messaging. So it would be switching.