Is there any coin mineable with unused cellular phone?
Yes... Take a look at Stellite and JSEcoin - Both are cell phone mineable
So, as Electroneum and any Cryptonight fork. I can't see any incentive to mine with cell phone.
I've accumulated several thousand JSEcoin coins mining on my phone. Not enough to retire (haha) but more than I had before. As long as you have unlimited data might as well accumulate some extra coins
You realize max supply is 10 Billion, right? Several thousands doesn't really mean anything and since you can't scale with phones(even if you would, that be stupid).