Central Checking

[poplar1]

Some wood wall phones were equipped with a pushbutton. Pushing the button while turning the crank connected one side of the generator to Ground and L2. Otherwise, the generator was connected to L1 and L2. When the generator was grounded, this allowed the caller to signal the operator without ringing the other subscribers on the party line. These phones were sometimes described as "Central Checking." This was common practice on Northern Electric phones, but not so much on Western Electric models.

Here is a Western Electric 1317 equipped with push button, stamped  SP 317 BA:

http://www.ebay.com/itm/361034993329

[Sargeguy]

I have heard it referred to as "Silent Signalling".  I have a TYPE 44 top box equipped with a switch hook that does this automatically when the receiver is lifted and a N.E.T&T 2-boxer that has the button.

[dsk]

In Norway on sidetone sets, it was common to have a button who had 2 functions. 
In on hook position; it would silence the ringer when cranking. (shortening ringers in series, or disconnect ringers in shunt.)  or
In off hook position; shorting the secondary af the induction coil, then the receiver became the only load between ring and tip, and your own transmitter would not disturb. (Nice on long bad lines, when listening only)

dsk

[davedolli]

See my post, Help identifying my new oak wall phone, on the first page now. My phone is set up with silent signaling.

Dave