The overheard call was disclosed on Monday by the top U.S diplomat in Ukraine, William Taylor

House Intelligence Committee Chairman Adam Schiff said on Monday Democrats have scheduled testimony from the person who said he overheard a telephone conversation in which President Donald Trump asked US ambassador to the European Union Gordon Sondland about investigations into the 2016 presidential election and the Bidens.

The overheard call was disclosed on Monday by the top U.S diplomat in Ukraine, William Taylor, during the first public impeachment hearings.

"I think this witness is potentially very important and of course are moving to depose this witness We have already scheduled the deposition," Schiff said without identifying the witness.

"What this call indicates, as other testimony has likewise indicated, is that the instructions are coming from the president on down," Schiff said after the hearings.