Question: Can a player pick a tile from the wall if another player says, “wait,” indicating they are thinking about taking the most recent discard for exposure?

Answer: Once a player has said, “wait,” or any other word or words indicating they may want a discard, the game stops until the player decides if they actually want the tile.

Source: Mah Jongg Made Easy edition 2020, pg. 14, THE PLAY

See ID 86 & ID89, Simultaneous call for exposure and for Mah Jongg