i dont think it will be efficient. because there are so well developped and complicated hardwares for mining. especially for bitcoin mining. i dont think it can be done in mobile phones. because they are such small devices.
I have the same point of view, my friend. However, last month I read that such smartphones will exist. Their price (the initial one) will be $999. Honestly, I do not think that this is the best idea.
I've never heard that there are such phones. Waste of money in mu opinion