In an interview to mark the 10th anniversary of the attacks, Cameron told Al Jazeera how he spent hours trying to contact Samantha.
Cameron was at the family home in Oxfordshire on the day of the attacks while Samantha - who was two months pregnant with their first son Ivan - was in New York opening a new branch of the upmarket stationers Smythson, where she still works as a consultant.
Although Samantha was in the shop a few kilometres from the World Trade Center, she was unable to tell her husband she was safe for five hours because the phone and mobile networks crashed in the immediate aftermath of the attacks, said the Daily Mail.
"Samantha was actually in New York that day, in Manhattan, and I'll never forget the hours of ringing her mobile over and over again and not being able to get through because the mobile phone system was down," Cameron said.
"I remember exactly where I was when I finally did get through and how pleased I was to hear her voice."
"Obviously it's been one of the defining events of this century. And I think we're still coming to terms with it and trying to get our response to it right," he said.