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.
Maybe this smartphone was made. But its purpose is only mining, because it is hard to imagine using it when the processor and the rest of the components work all the time with full power.