You're about a year late since the last time it was worth buying cheap smartphones to mine with. By cheap, that's $20 or less per phone. If you started back then or earlier, it's worth keeping it running (as I do right now), but it's not worth getting started now.

Smartphone CPUs can do better in performance per watt compared to desktop CPUs. They can't compete with GPUs or ASICs, which is why smartphone coins are GPU and ASIC resistant.