Phone hacking, being a form of surveillance, is illegal in many countries unless it is carried out as lawful interception by a government agency. In the News International phone hacking scandal, private investigator Glenn Mulcaire was found to have violated the Regulation of Investigatory Powers Act 2000. He was sentenced to six months in prison in January 2007. Renewed controversy over the phone hacking claims led to the closure of the News of the World in July 2011.

Is it true that is it illegal to hack into someones phone?
yes