Former President Joseph R. Biden Jr.’s aides did not want him to speak with me.

For months, as I worked on a book about the 2024 presidential election, I made multiple requests for an interview with Mr. Biden. One of my co-authors had sat down with President-elect Donald J. Trump, and we felt it was critical to talk to Mr. Biden. But the former president’s aides said he was working on a memoir, and that would conflict with my book.

Yet when I reached Mr. Biden on his cellphone in late March, he answered and agreed to talk. He broke his silence on his successor to criticize the early weeks of Mr. Trump’s second term. “I don’t see anything he’s done that’s been productive,” the former president said.

When I asked if he had any regrets about dropping out of the presidential race, Mr. Biden said, in a detached tone, “No, not now. I don’t spend a lot of time on regrets.” Then he hung up because he was boarding an Amtrak train.

My brief conversation with Mr. Biden prompted a cascade of concern among his top aides. One screamed at me for calling the former president directly. Others texted furiously, trying to figure out how I had obtained Mr. Biden’s phone number.

Mr. Biden had seemed open to continuing the conversation, but my subsequent calls went straight to voice mail. His automated greeting simply said, “Joe.”

Two days later, that greeting was replaced by a message from Verizon Wireless: “The number you dialed has been changed, disconnected or is no longer in service.”