Fewer administration officials are allowed to listen to President Trump Donald John TrumpBiden says voters should choose who nominates Supreme Court justice Trump, Biden will not shake hands at first debate due to COVID-19 Pelosi: Trump Supreme Court pick 'threatens' Affordable Care Act MORE's phone calls with foreign leaders after the president's July 25 call with the leader of Ukraine became a central part of the House impeachment inquiry, CNN reported, citing White House sources.

Transcripts of the calls are also being given to a smaller number of officials, sources told the news network.

"Nobody is allowed on the calls," a White House official told CNN while describing a new attempt to limit access to the calls to top aides only. "The barn door officially closed after the horse escaped."

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Multiple officials told CNN that when Trump speaks with world leaders, only a select few are on the call, and the list is approved by national security adviser Robert O'Brien.

Previously in the administration and during other presidencies, a greater number of officials were reportedly permitted to listen in, including aides with expertise on the countries involved in the calls.

One official jokingly referred to the change as "The Vindman Rule," a reference to Lt. Col. Alexander Vindman Alexander VindmanImpeachment witness Alexander Vindman calls Trump Putin's 'useful idiot' The Hill's Morning Report - Sponsored by The Air Line Pilots Association - Trump, Biden renew push for Latino support Strzok: Trump behaving like an authoritarian MORE, who listened to the July 25 call and reported it to National Security Council lawyers.