Seat the students in single desks facing the board or teacher for silent work.
Move them to another part of the room to talk. This could be a carpeted area or a seating circle. Sure, classroom space will be a limiting factor but you can be creative.
When you call the students back to their chairs, ensure that they know that it is a NO-TALK zone.
This behaviour management strategy is a bit like Pavlov's dogs theory of classical conditioning. You are training the students to associate sitting down in the chairs with listening.
Wouldn't it be great if it was that easy.
This strategy can be used with any trigger. Maybe a small flag up could be used as a listening signal. You could experiment with what ever suits you.