Allowlists for VoLTE are a perfectly reasonable (if extremely unfortunate) safety measure.
Because of the special handling that emergency calls must get in cellular networks, many older phones will use circuit-switched fallback (AKA 3g or 2G) for those calls, even if they are otherwise VoLTE capable.
This was pretty much fine back in the day, as 2g and 3g networks were just as (if not more) widespread as VoLTE, but those networks are now being shut down. If you have one of these phones and an LTE-only carrier allows it onto their network, you will be able to make any normal call, but emergency calls will not work.
To add insult to injury, there's no way for the carrier to tell whether any specific phone does emergency calls over VoLTE or not, especially if they don't have a contract with that vendor. Some phones may only do it in certain configurations, E.G. when branded for that specific carrier and configured with their preferred modem settings.
Because of the special handling that emergency calls must get in cellular networks, many older phones will use circuit-switched fallback (AKA 3g or 2G) for those calls, even if they are otherwise VoLTE capable.
This was pretty much fine back in the day, as 2g and 3g networks were just as (if not more) widespread as VoLTE, but those networks are now being shut down. If you have one of these phones and an LTE-only carrier allows it onto their network, you will be able to make any normal call, but emergency calls will not work.
To add insult to injury, there's no way for the carrier to tell whether any specific phone does emergency calls over VoLTE or not, especially if they don't have a contract with that vendor. Some phones may only do it in certain configurations, E.G. when branded for that specific carrier and configured with their preferred modem settings.