Former Defense Secretary Jim Mattis put a phone call with Vice President Mike Pence on hold as he was volunteering at a food bank, according to an upcoming book written by Mattis' aide.

Guy Snodgrass, a retired US Navy commander and Mattis' former speechwriter, recounts in his book, "Holding the Line: Inside Trump's Pentagon with Secretary Mattis," a story the defense secretary shared with his aides during his two-year tenure.

Mattis was moving crates when the call came through from the vice president-elect, Snodgrass wrote in his book. "Hi Jim, this is Mike Pence," Mattis recounted Pence saying.

Mattis was busy with his tasks and not in a suitable location to talk: "No, not at the moment. I'm right in the middle of something. Can I call you back?"

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Former Defense Secretary Jim Mattis put a phone call with Vice President Mike Pence on hold as he was volunteering at a food bank, according to an upcoming book written by Mattis' aide.

Guy Snodgrass, a retired US Navy commander and Mattis' former speechwriter, recounts in his book, "Holding the Line: Inside Trump's Pentagon with Secretary Mattis," a story the defense secretary shared with his aides during his two-year tenure.

Mattis, who officially retired from his four-star general post in 2012, received a phone call when he was busy volunteering at the Tri-Cities Food Bank in Washington, shortly after the 2016 US presidential election.

Mattis was moving crates when the call came through from the vice president-elect, Snodgrass wrote in his book.

"Hi Jim, this is Mike Pence," Mattis recounted Pence as saying, according to Snodgrass. "Do you have a minute to chat?"

Mattis was busy and not in a suitable location to talk: "No, not at the moment. I'm right in the middle of something. Can I call you back?"

Read more: 'Who gives a s--- about Afghanistan?': Trump stunned officials with his comments during a military briefing, former aide says

Trump and Mattis in 2018. AP Photo/Manuel Balce Ceneta

Mattis would later contact Pence, who informed him he was on the list of Trump's potential defense-secretary picks. Pence asked him if he could make a trip to Trump's golf club in Bedminster, New Jersey, a trip he made on November 19, 2016.

Before making the trip, Mattis was said to have spoken with friends and family about the decision. Following the meeting, Trump praised Mattis and hailed him as "the real deal."

"He's just a brilliant, wonderful man. What a career," Trump said to reporters, with Mattis in tow. "And we're going to see what happens."

But underneath the formalities, Mattis privately told his aides he "disagreed with the president-elect on every one of the main points that he raised" in the meeting. Mattis believed he left such a lackluster impression on Trump that he "certainly won't be hearing back from those guys," Snodgrass wrote.

In one exchange, Mattis disagreed with Trump's positive view of the controversial use of waterboarding against prisoners of war.

"I've never found it to be useful," Mattis said, according to Trump. "Give me a pack of cigarettes and a couple of beers, and I do better with that than I do with torture."

Mattis would eventually become the 26th US defense secretary on January 20, 2017. Mattis fell short of a federal regulation requiring nominees to be out of the military for seven years and needed a waiver for the confirmation process.

After the House and Senate approved his waiver, the upper chamber overwhelmingly green-lighted his nomination in a 98-1 vote. Democratic Sen. Kirsten Gillibrand of New York was the sole lawmaker to oppose his nomination.

Mattis resigned in December after serving for two years. In his resignation letter, Mattis said he was no longer able to carry out Trump's policies and that Trump, as the commander in chief, was entitled to a defense secretary "whose views are better aligned."