Hello David
Telephone Loudspeaker was used to control a battery of 25 pdr's or 4.2" mortars.
The L.S. No.2 Canadian Control Unit was located at the CP and connected to the Loudspeakers by 2 wire cable. The CP could send instructions to the guns and the guns could reply by pressing the Push button and then yelling into to speaker to send a reply.
Some scans from a 1943 British 4.2" Mortar Manual.
Geoff
