It won't happen.
Smartphone for bitcoin mining? you are just destroying your phone.
Smartphone as digital wallet, you can simply download an app / wallet and that's how you make it as your wallet.
You never know! With the help of nano technology we may be able to achieve smaller chips that produces as much power as a full sized mining rigs in future. I mean that is not the reality now but it may become in future. If you were 80's kid, did you ever think that you can carry a computer in your pocket? Because what computer we have seen that time was Pentium one and it required an air conditioned room to run that. Nano technology achieved that impossible and now we carry a smart phone inside our pocket.
So keep your hope high as we don't know where the technological achievements can take us!
Technology had come so far, the things that we think impossible back then, it is actually existing now. Mobile phones is not different in computers it is just handy and portable. Computers are more appropriate to use than laptops ,(
because it's electronic parts are small and really makes the temperature go high even the processing range can do mining but the electronic materials can't) , even now laptop are not yet used for mining but as technology evolves in a different way, for me it is not impossible to use laptops and mobile phone on mining in the near future.