William said he wants people to speak more openly about mental health.
"It was only right to share my experiences to hope to encourage others to come forward and smash that stigma, to make it easier for them to talk about their own experiences - so I was just doing my bit". It showed Lady Gaga taking the call in her home, while Prince William was at his desk in Kensington Palace.
Lady Gaga - whose real name is Stefani Germanotta - spoke about her struggle with post-traumatic stress disorder in an open letter for the first time last December.
The pair also made plans to meet when Lady Gaga comes to the United Kingdom in October, and the singer said: "We have to make the strongest, most relentless attempt we can to normalise mental health issues so that people feel like they can come forward".
"But you can't help it if in the morning when you wake up, you are so exhausted, you are so sad and you are so full of anxiety and the shakes that you can barely think", she continued. "But [writing the letter] was like saying, this is a part of me, and that's OK". "It's the same as physical health".
The Duke of Cambridge made the comments in an upcoming BBC documentary Mind Over Marathon, which airs tomorrow night.
It comes after Prime Minister Theresa May praised Prince Harry for his bravery in revealing that he sought counselling to come to terms with the death of his mother, Diana, Princess of Wales. "You just learn to deal with it", he said.
Harry, filled with anxiety and anger, acknowledged that he went through two years of "total chaos" in his 20s.
"Because of the process I have been through over the past two and a half years, I've now been able to take my work seriously, been able to take my private life seriously as well, and been able to put blood, sweat, and tears into the things that really make a difference and things that I think will make a difference to everybody else", he added.
Although Lady Gaga has won three Brit Awards, six Grammy Awards and a Golden Globe gong for her role in American Horror Story: Hotel, her mental illness left her feeling sad and full of anxiety because she could not appreciate the "wonderful things" she owned or the successes she had achieved.