Image s
Phones do thing like kill or pause running processes when the screen turns off, or when they're not the current fullscreen app. They throttle down the CPU when you're not actively interacting with them (eg, by using the touchscreen) to save power, they almost always run slower on battery than on charge, they aggressively slow things down to manage heat (a red hot CPU next to a lithium battery is a Bad Idea), all of which will either stop mining or slow down mining so much as to make it pointless.
You could possibly use stripped down phones without batteries, with casing removed and forced air cooling, with custom operating systems to run at full capacity... But at that point you might as well use dedicated embedded arm systems instead (and either way you'd be using far more power than the mined coins would be worth
