On my carrier it's called a MultiSIM. It's having two SIM cards with the same number. On most carriers you can set up if you want this extra SIM to have voice or data (or both). It's usually cheap.

It's true that having two devices might seem complicated, but this is the only setup that ended working out for me: when I know I won't need any smart features on my life, I am happy to go out with my dumphone without worrying about missing urgent calls.

smartwatches use those often, so perhaps that's something your carrier will have heard of / offer as a service