After Prince William and Prince Harry discussed their regret at the brevity of their final phone call with their mother, Princess Diana, Franklin Graham explained why the sobering event "should motivate us to action."

Graham, the founder of Samaritan's Purse and the son of evangelist Billy Graham, said Princess Diana's shocking death should remind us that "none of us know when time on earth will end for us, or for those around us."

"That reality should motivate us to action," he said. "How many opportunities do we miss to tell those we love how much they mean to us? Is there someone you need to check on or spend time with today? Is there someone you need to tell they are cared about? Is there someone who you need to tell that Jesus Christ loves them and gave His life so they could be saved? Now's your chance!"

In a new documentary set to release on Monday by ITV in Great Britain and HBO in the United States, Prince Harry recalls how he quickly got off the phone with his mother just hours before her 1997 death in a car crash in Paris.

"I can't necessarily remember what I said, but all I do remember is regretting for the rest of my life how short the phone call was," Harry told ITV.

"If I'd known that that was the last time I was going to speak to my mother, the things I would have said to her," he said. "Looking back at it now -- it's incredibly hard. I have to deal with that for the rest of my life: not knowing that it was the last time I'd speak to my mum, how differently that conversation would have panned out if I'd had even the slightest inkling that her life was going to be taken that night."

In the documentary, Prince William said he and his brother, at the time ages 12 and 15 respectively, had been "running around, minding our own business, playing with our cousins and having a very good time" and were in a rush to say "goodbye, see you later, can I go off?"

"If I'd known what was going to happen I wouldn't have been quite so blasé about it," the Duke of Cambridge said. "That phone call sticks in my mind quite heavily."

The ITV documentary, "Diana, Our Mother: Her Life and Legacy," celebrates the life and work of Diana, Princess of Wales, in the 20th year since her death, according to a press release by the network. It features candid conversations with the princes and Diana's friends, family and admirers.

The documentary is executive produced by Nick Kent of Oxford Film and Television and by producer-director Ashley Gething. For HBO, Nancy Abraham serves as the film's senior producer, with Sheila Nevins as its executive producer.