> What should I be looking for with regards to a VoIP box?

I just bought the one from Ubiquiti. No fuss, works out of the box: https://store.ui.com/us/en/category/managed-voip/products/ut..., though you do need a separate piece of hardware on which to run the UniFi talk app. For me, that was my UniFi gateway (UXG-Max) - I have a lot of UniFi equipment.

There are others that could work - you can look up UniFi Talk supported devices.