Graham Foulkes, whose son David died during the July 7 bombings, told ITV News he was taken "back to that time" when a police officer told him his phone may have been hacked, calling it "unbelievably wicked".
Mr Foulkes said his family "left some very personal and very emotional messages" on the answerphone of David's mobile during the days that he was missing.
His 22-year-old son was one of the 52 people killed in the 2005 terror attacks on London.
David Foulkes had travelled to the capital to attend work meetings after London won the bid to hold the 2012 Olympic Games.
Asking his father the best way to get around the city, Mr Foulkes told him to get off the train at Euston and take the Underground three stops before getting off.
David was standing within a few feet of the Edgware Road bomber Mohammad Sidique Khan when the device detonated.
Describing the moment he found out his phone may have been hacked, Mr Foulkes said:
That week when we had no information about David and we were leaving messages on the answerphone was not a time that I wanted to be reminded of. It was a very difficult and emotional and dark place. And so to be told five years later when we'd just had the inquest, when we were given the full details of how David had died ... for a police officer to phone us up and say that this private investigator had my private details and David's phone numbers ... it took me back to that time.
Mr Foulkes told ITV News, "To potentially intrude into somebody's life when they're at their darkest and most desperate time for the sake of headlines is a most horrendous crime".
He continued: "I struggle to understand how these people could live with themselves.
"They must have been aware that it is wrong - nevermind whether it's legal or illegal, morally it's wrong.
"I don't have a very high opinion of these people - they were doing it for pounds, shilling and pence, and they had no feelings or sensitivity about the people they were damaging."