Now that I have no more landline or VoIP phones, I keep all my old smartphones in strategic places around the house, in case I might need to make an emergency call. One downside to living in gun country is that your neighbor is far less likely to go in your house to help. I miss calls all the time because my main phone is upstairs when I'm downstairs & vice versa, so I'd hate to hear something go bump in the night and realize that my only phone is out where the burglars are. It's also nice to have a not-too-old phone as a backup in case I leave my main phone somewhere by accident. With Google Fi I can get my old phone up and running on my usual number until I'm reunited with my newest phone.
I used my old Galaxy SII (3G plus WiMAX) as a backup phone, with Ting service until the phone bricked itself right after a software update. Pity. I have a number of Verizon devices including a Droid Turbo 2, an OG Pixel XL and a RED Hydrogen One. I'm hoping that the RED phone will become a collectible item one day (I've already had interest from a local phone phan), but the others are in working condition, so why not use them for something?
Ting was a solid MVNO for my old Sprint phone; now that they also use T-Mobile (the most reliable carrier in my area), paying $6/mo./phone seems a reasonable cost to keep 'em all online just in case. If I had an old phone on every major carrier, that could do at least voice & text, that would be a pretty fault-tolerant system.