The V-2971 (Call Switch) is designed to be used with the V-2924A and V-2925A Intercom Units. When the black button is pressed, the V-2971 produce a momentary closure for calling into the V-2924A.Use with V-2924A control unit.Fits a single-gang electrical box.Brushed stainless steel wall plate.Black ''call'' button for normal call.Built-in volume control.Single pole momentary push button.Brushed stainless steel singlegang wall plate with black screen printed text.MOUNTING : Call button mounts in a single gang electrical box using two screws.Dimensions: 4.5 ''H x 2.75 ''W x 1.0 ''D (11.43 cm x 6.985 cm x 2.54 cm).