The small sensor and optics on a smartphone are not optimal for astrophotography, but you can get around that - sort of. Andrew Symes has made remarkable progress. As you might expect there is a lot of image stacking.
I've used the NightCap app on my iPhone 6 Plus. It works reasonably well if the camera is very stable. We'll probably see multi-image sensor smartphones in a year or two. I'm more interested in the three sensor/dichroic filter approach, but spider eyed cameras seem likely.