Check out USMobile.
They're a VMNO but they use every network.
At home I get LTE, when I leave I'm immediately on Verizon 5G. I've travelled across the US and watched it change from ATT to Verizon as I went, often on 5G.
I pay ~$25/mo for 2 lines with 4GB data. You can probably cut it a little since you don't need data.
Alternatively check out jmp.chat - I use them so my SMS gets piped into XMPP, but it's also really cheap (less than $10/mo) and adding voice minutes is crazy cheap, like tenths of a cent per minute or something. Minutes/data don't expire - you can use as many minutes/data as you have money for in your account (again, it's so cheap that I don't care it's a pay first setup) and it can auto-bill if you run out of credits (if you choose to).