On Wednesday, the royal author appeared on the British TV program "Lorraine" where he shared some insight about Philip based on 40 years of friendship.
"He said to me, ‘God, she’s never off the phone. Never off the phone. Who is she talking to?’" Brandreth recalled, as quoted by U.K.’s DailyMail.
"It was the only time he ever complained about the queen," Brandreth shared. "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."
The biographer shared that Elizabeth often rang her racing manager to talk about her horses, one of her passions, during the evenings.
Philip, who spent more than seven decades supporting his wife, passed away on April 9 at age 99. He is known as Britain’s longest-serving consort.
In his lifetime, Philip fulfilled more than 20,000 royal engagements to boost British interests at home and abroad. He headed hundreds of charities, founded programs that helped British schoolchildren participate in challenging outdoor adventures and played a prominent part in raising his four children, including his eldest son, Prince Charles, the heir to the throne.
The queen, a very private person who isn't known for giving extravagant displays of affection, once called him "her rock" in public. Behind palace doors, Philip called his wife Lilibet; but he referred to her in conversation with others as "The Queen."
When Elizabeth was crowned queen in 1953, Philip had no desire to enjoy a life of ease and wealth. Instead, he promoted British industry and science, espoused environmental preservation long before it became fashionable, and traveled widely and frequently in support of his many charities.
In those frequent public appearances, Philip developed a reputation for being impatient and demanding and was sometimes blunt to the point of rudeness.
When Philip turned 90 in 2011, the royal told the BBC he was "winding down," adding, he had "done my bit." It wouldn’t be until 2017 when Philip announced he planned to step back from royal duties.
Philip is survived by the queen and their four children — Prince Charles, Princess Anne, Prince Andrew and Prince Edward — as well as eight grandchildren and 10 great-grandchildren.
The Associated Press contributed to this report.