President Trump’s assertion that his phones at Trump Tower were tapped last year has been treated as hilarious—and in some circles as beyond contempt. But I can vouch for the fact that extracurricular surveillance does occur, regardless of whether it is officially approved. I was wiretapped in 2011 after taking a phone call in my congressional office from a foreign leader.

That a secret recording had been made of this call was revealed to me by the Washington Times in 2015, a full two years after I left office.

The newspaper’s investigative reporters called me, saying they had obtained a tape of a sensitive telephone conversation that they wanted me to verify.

When I met them at a Chinese restaurant in Washington, they played back audio of a call I had taken in my D.C. congressional office four years earlier.

The call had been from Saif el-Islam Qaddafi, a high-ranking official in Libya’s government and a son of the country’s ruler, Moammar Qaddafi.

At the time I was leading efforts in the House to challenge the Obama administration’s war against Libya. The Qaddafi government reached out to me because its appeals to the White House and the State Department to forestall the escalating aggression had gone unanswered.

Before taking the call, I checked with the House’s general counsel to ensure that such a discussion by a member of Congress with a foreign power was permitted by law.

I was assured that under the Constitution a lawmaker had a fundamental duty to ask questions and gather information—activity expressly protected by the Article I clauses covering separation of powers and congressional speech and debate. I could and did ask questions of the younger Mr. Qaddafi.

On the Libyan end, the risk of the conversation was that whatever phone was used to call my office might serve as a homing beacon for a drone strike.

That possibility was minimized, I was told, by calling me on a cellphone that was used only once and then discarded.

Somehow, the Washington Times had gotten its hands on the surreptitious recording. I authenticated the conversation, and parts of it were published by the newspaper, which provided online links where readers could listen to me talking with Mr. Qaddafi.

The reporters did not say, nor did I ask, who had made the tape. But the paper’s stories referenced “secret audio recordings recovered from Tripoli.”

I have only my suspicions about their true provenance. The quality of the recordings was excellent on both ends of the call.

If sources had indeed discovered the tapes in Tripoli, there is no plausible explanation for how they would have chosen the Washington Times to carry the story. And which foreign intelligence service conceivably could have been interested in my phone call, had the technology to intercept it, and then wanted to leak it to the newspaper?

There’s a simpler explanation: I believe the tape was made by an American intelligence agency and then leaked to the Times for political reasons. If so, this episode represented a gross violation of the separation of powers.

Shortly after the Times story was published, I alerted congressional leaders to the breach and then let the matter rest, assuming that a series of routine Freedom of Information Act requests I had made in 2012 before leaving office would provide answers.

Five years later I am still waiting for FOIA responses from some of the intelligence agencies.

I cannot say with assurance that my Libya call was the only one intercepted.

I have never gone public with this story, but when I saw the derision with which President Trump’s claims were greeted—and notwithstanding our political differences—I felt I should share my experience.

When the president raised the question of wiretapping on his phones in Trump Tower, he was challenged to prove that such a thing could happen.

It happened to me.