The program is scheduled to air Aug. 31 to coincide with the 20th anniversary of Diana's death in a Paris car crash at the age of 36.
William and Harry -- who were 15 and 12 respectively at the time -- said they cut short the phone call with their mom because they were eager to go back to playing with their cousins at their grandmother the queen's Scottish estate, Balmoral.
"And if I'd known that that was the last time I was going to speak to my mother the things that I would -- the things I would have said to her," Harry says in the TV special.
Kensington Palace tweeted out several, rare, private photos of the princes and their mother this weekend, as well.
"The Duke and Prince Harry are pleased to share 3 photographs from the personal photo album of the late Diana, Princess of Wales," the palace captioned the images.
The Duke and Prince Harry are pleased to share 3 photographs from the personal photo album of the late Diana, Princess of Wales. pic.twitter.com/mUhzQB08TV— Kensington Palace (@KensingtonRoyal) July 23, 2017