Verizon prevented two of my unlocked phones (Nexus 6P and Moto G6 Power) from using WiFi calling, likely because I didn't buy them through them. The phones were perfectly capable of it but it was just some carrier config that prevented it. I bought an unlocked Samsung A53 and that is suddenly allowed to use WiFi calling, likely because Verizon sells the exact same model. I went years without WiFi calling despite living in two homes that had awful cell signal. Now I have a phone with the feature but have great home cell signal.

I concur, but carriers do weird things. Some of them will want you to purchase a more expensive plan to get that “feature.”

Cricket only allows WiFi calling on certain devices. I had to switch to Mint to get it working

How do they prevent wifi calling?

WiFi calling in this context doesn't mean any random VoIP app, it means using your same phone number as the cellular service uses. That requires the carrier to be involved on the server side even if your handset is not using the cellular modem. (And it probably is using the cellular modem to some extent even during a WiFi call, to support handover between WiFi and cellular.)