Custom firmware is not required.
However, carriers do whitelist phones and their VoLTE/VoWiFi implementations, which is quite unfortunate and I whish it wasn’t the case.
That said, it is their IMS and their choice which clients they allow.
There likely are providers that do not use whitelists (though they are likely rare), but more importantly, there are 3rd party IMS implementations, including open source ones, which could be used with the VoWiF clients on any phone, which certainly qualifies it as an open standard, and certainly is quite an advantage over “Republic”, where no open source implementation exists on either end.
Edit: it also should be noted, that an opensource VoWiFi client could be developed and might be certified by various carriers free of charge if it works correctly, which is an approach that certainly should be attempted.