VoLTE seems to be a more difficult service to be implemented in roaming conditions. Since most cell carries prefer to have their own way of implementing it, making it seamless in between carries can be quite a daunting challenge. So carriers avoid leveraging this whilst roaming.
The only exception I can see is the US. Since you have no other choice, but to connect to their 4G and 5G infrastructure. In Europe it's not the case. 2G will stay for a long time, up till 2030 probably.
As a side note, you could try turning off Wi-Fi calling whilst abroad. You may be surprised to know that this can actually interfere with your service while enjoying your vacation abroad. Your cell carries may think you're still at home, and charge you more than what you might expect, especially if you're connected to a VPN with its endpoint in your home country.
Second, as some people stated above on the topic, avoid 4G/LTE only mode. Normal calls would still require 3G or 2G anyway, and there's quite nothing we can do about it. You could use Internet calls (Signal) to avoid such downgrade though, but I don't know how affordable roaming costs would be for any of you, so beware.
And lastly, make sure you actually have roaming service from your carrier. Perhaps that would have been the cause of having no service at all?