Dumbphones are 4G. 4G voice service requires VoLTE but mobile operators have to whitelist phone models, the phone has to have the IMS in the internal database... all in all don't expect VoLTE to work. So voice downgrades to 3G. But 3G is being shut down by many operators and more to come. So voice downgrades to 2G, but 2G has a very low limit of concurrent calls, let's say a few dozen. So it's possible that your dumbphone won't work at all.