Prince Philip only had "one complaint" about his wife Queen Elizabeth throughout their marriage of 73 years.
The Duke of Edinburgh would complain about the monarch's habit of "always being on the phone", his biographer says, according to Fox News.
Royal author Gyles Brandreth appeared on UK TV show Lorraine to share his insights into Philip's marriage and his 40-year friendship with the duke.
"He said to me, 'God, she's never off the phone. Never off the phone. Who is she talking to?'" Brandreth said.
And he went on to say that it was the only time he ever complained about the Queen.
"He knew his whole life was supporting the Queen. He never put a foot wrong, he always turned up right day, right uniform, on time, one step behind her."
Brandreth said that the Queen often rang her racing manager in the evenings to talk about her horses, one of her greatest passions.
Philip, who was the Queen's strongest supporter for more than 70 years, died on April 9 aged 99. He was Britain's longest-serving consort.
During this life, Philip went to more than 20,000 royal engagements, boosting British interests locally and overseas. As well as heading hundreds of charities and founding programmes that helped British children undertake challenging outdoor activities, he played a huge role in raising his four children.
The Queen is very private and known for not displaying her emotions, once called him her "rock" in public. Philip called her "Lilibet" but in conversation with others, referred to her as "The Queen".
When Elizabeth was first crowned Queen in 1953, her husband had no wish to simply sit back and enjoy a life of riches. Instead, he threw his energy into promoting British industry and science, became an environmental advocate well ahead of his time, and travelled all over the world in pursuit of different charity projects.
He became known for being impatient and demanding during public appearances, sometimes to the point of rudeness.
When he turned 90 in 2011, he told the BBC he was planning to "wind down" as he had "done his bit". But it wasn't until 2017, just four years ago, that Philip announced his plans to step back from royal service.
He is survived by Queen Elizabeth, 95, and their four children, Charles, Anne, Andrew and Edward - as well as eight grandchildren and 10 great-grandchildren.