Randall Stephenson, the CEO of AT&T, addressed the Michael Cohen scandal in a memo to staff on Friday.

The memo said AT&T made a "big mistake" by hiring Cohen, President Donald Trump's attorney, to get advice about the Trump administration's thinking.

Stephenson said AT&T's chief lobbyist, Bob Quinn, was retiring, though a source told The Wall Street Journal he was forced out.

AT&T is one of several companies that paid Cohen, along with the pharmaceutical giant Novartis and Korea Aerospace Industries.

The CEO of AT&T on Friday told his employees that the company made a "big mistake" by hiring Michael Cohen to get insider insight into the Trump administration — a decision said to have cost a top exec his job.

In a memo to staff obtained by Reuters, Randall Stephenson said AT&T made a "serious misjudgment" by paying Cohen as much as $600,000 via Cohen's company Essential Consultants LLC.

Stephenson also said Bob Quinn, AT&T's chief lobbyist who oversaw Cohen's contract with the company, would retire. Citing a person familiar with the matter, The Wall Street Journal reported that Quinn was forced out.

Cohen's political payments, which also came from Novartis and Korea Aerospace Industries, have been increasingly scrutinized in recent days and may attract the attention of investigators.

AT&T CEO Randall Stephenson at a court hearing in April. Reuters

Stephenson wrote: "Our company has been in the headlines for all the wrong reasons these last few days and our reputation has been damaged. There is no other way to say it — AT&T hiring Michael Cohen as a political consultant was a big mistake."

Stephenson took responsibility for what he described as a failure in the vetting process that led to Cohen's hiring.

AT&T said on Tuesday it hired Essential Consultants to advise it on working with the new administration in early 2017, around the time of Trump's inauguration.

"To be clear, everything we did was done according to the law and entirely legitimate," Stephenson wrote in the memo. "But the fact is our past association with Cohen was a serious misjudgment."

AT&T did not hire Cohen to lobby on behalf of the company, the memo said. The distinction could be important if Cohen's activities become the focus of a criminal investigation.

Cohen's one-year contract with AT&T — $50,000 a month from January to December 2017 — was limited to consulting and advisory services, according to the memo.

The payments by AT&T and others were revealed by Michael Avenatti, an attorney representing the adult-film actress Stormy Daniels. Avenatti also said that among the entities that paid Cohen through Essential Consultants was a company owned by the Russian oligarch Viktor Vekselberg.

Essential Consultants paid $130,000 to Daniels, whose real name is Stephanie Clifford, days before the 2016 presidential election as part of a nondisclosure agreement that barred her from discussing an alleged sexual encounter with Trump. He denies any encounter took place.