Granted, I knew this was coming. I'd heard from better informed people than myself that although my phone works on Vodafone's network, which had switched off it's 3G support in January, it would not work on Telstra's network when it came to decommission it's 3G.
My (limited) understanding is that early iterations of the 4G standard would simultaneously rely on 3G when doing things like transmitting voice and data at the same time. So without 3G available, you need a capability called VoLTE . My phone can do it. I emailed the manufacturer and got a copy of the latest modem driver from them, so I am — technically at least — officially future-proof.