Something like this do? https://www.amazon.com/Nokia-3310-3G-Unlocked-T-Mobile/dp/B075FL4H89
An external battery has a massive standby time and can quickly charge up a phone. You'd have to charge it, eventually, just as you'd have to charge a 2nd cellphone. But, it doesn't require a plan or dealing with another number and such.
But, whatever. https://en.wikipedia.org/wiki/Rube_Goldberg_machine