The iPhone 5s can only access LTE for data and is not VoLTE capable. It will no longer work for cellular calls after the networks completely phase out 3G UMTS. However, I have a hypothesis that a 5s that is fully updated to IOS 12 might still work for wifi calling even after 3G networks are retired. I am basing this on the fact that a 5s I currently own is wifi calling capable on Mint even though its 3G antenna is defective.
Thoughts???
The question may be whether anyone will allow it to be activated. Seems like both Verizon and AT&T are getting pretty aggressive about kicking phones off the network. T-Mobile remains more flexible for now, but I imagine they will also enforce VoLTE requirements when only LTE and 5G are available.
Verizon and their MVNOs definitely don't allow you to do initial activation on a non-VoLTE phone. They haven't allowed that in almost two years. However, I have generally not had trouble swapping an already activated SIM into a non-VoLTE phone. I've done this successfully with my post-paid Verizon SIM that work gives me, as well as on Xfinity. But I did have an issue SIM-swapping on Total Wireless earlier this year.
For AT&T I only know that my legacy free FreeUp talk/text SIMs work on non-whitelisted phones. In fact, one of them is in a Verizon-issued Galaxy S5 on which I haven't even bothered manually inputting the APN.