Phone hacking is the practice of exploring a mobile device, often using computer exploits to analyze everything from the lowest memory and CPU levels up to the highest file system and process levels. Modern open source tooling has become fairly sophisticated to be able to "hook" into individual functions within any running app on an unlocked device and allow deep inspection and modification of its functions.
Phone hacking is a large branch of computer security that includes studying various situations exactly how attackers use security exploits to gain some level of access to a mobile device in a variety of situations and presumed access levels.
The term came to prominence during the News International phone hacking scandal, in which it was alleged (and in some cases proved in court) that the British tabloid newspaper the News of the World had been involved in the interception of voicemail messages of the British royal family, other public figures, and murdered schoolgirl Milly Dowler.[1]