CLAIMING A DISCARD MUST BE VERBALIZED (Article 57)

Commentary: When a player is interested in a discard, they must verbalize it before picking it up. Sometimes a player is undecided or may not know for certain if they need the tile, so they may say, “Wait” or “Hold.” The other players should give them reasonable time to think when this happens. Reasonable time for decision-making depends on the format (e.g., social, competitive) and the group culture (e.g., casual, competitive). Learn more in this wiki article: GUIDELINES FOR DECISION-MAKING TIME (Article 198).

Question from a player:  A player discards a tile.  The next player in turn wants to call that discard for exposure – does not say anything – and simply picks up the discard and makes her exposure. 
 
Answer from the NMJL:  As soon as a tile is discarded, any player wanting that tile for an exposure may claim it, preference being given to the player next in turn, unless another player has called and exposed. To claim a discard the player must verbalize their call by saying “call,” “take, “I want that,” etc., letting the other players know that they are claiming the discard. It is permissible to say “wait” or “hold” before verbalizing a call. A caller is committed to taking a tile once the called tile has been placed on top of the rack or tiles have been exposed from the hand.
 
Source:  Mah Jongg Made Easy (2024 page 14-15)

Source for other answers: National Mah Jongg League Bulletin Q&A (1983 page 4, 1984, 1985, 1987 page 4, 1990 page 5, 1993 page 12, 2005, 2019)