The technology within the phone hacked. Its been around for ages. The royal family members phones were hacked!
I am told by a technician one of the only hints you will get is your phone's battery will run out of charge faster than normal.
https://www.techlicious.com/tip/how-to-tell-if-your-phone-has-been-hacked/
There is loads of info online. Google...phone hacking listening to personal conversations.
Edited to add.....The hacking technology is loaded onto the phone to be hacked. All the hacker requires is the phone number. MFC mobile number was widely publicized through his work I am informed. Photographs of his vehicle and rego number were posted by him in photos online 12 days before William's abduction.