One option that tends toward this is to have a cellular modem which tethers to the device of your choice. SIM is assigned to that device. Voice comms go over

One option that tends toward this is to have a cellular modem which tethers to the device of your choice. SIM is assigned to that device. Voice comms go over a VOIP service such as Jitsi or whatevs. Carry the VOIP device you want to use, whether a minimal comms-only device, or a more capable tablet or laptop which you can talk to. A device which talks to a headset or transport-based option (e.g., your car's Bluetooth system) might also be convenient. The idea that you need a full-compute device with a high-res screen just to make voice calls is ... somewhat silly. The main concern I have with this is the level of phone spam either now present or in the likely future. You'll want a comms system which filters out unknown and/or suspect calls. PSTN, of any stripe (POTS, mobile, VOIP) isn't cutting it any more.