This article talks about the different sources of phone hacking.  malicious apps, phishing schemes, operating system vulnerabilities, clicking on malicious links etc.  The article has interesting pieces of information like the fact that about half of the discovered vulnerabilities were not found int the phone's OS but in the software that run the chips (firmware).     

https://techcrunch.com/2016/10/08/why-an-unhackable-mobile-phone-is-a-complete-marketing-myth/