Gartner has recorded a 5.6 percent decline in smartphone sales from Q4 2016 to Q4 2017. Key reasons given include a lack of good budget smartphone options and owners of more expensive phones opting for a longer phone lifecycle before upgrading.
https://www.notebookcheck.net/Q4-2017-brought-the-first-ever-decline-in-smartphone-sales-according-to-Gartner.285152.0.html