What does that white button actually do? Almost nobody knows

Klassiek grijs telefoontoestel met aardtoets
Klassiek grijs telefoontoestel met aardtoets
White button
White button

Have you ever pressed one? I have. Many times. Way back when, as a kid. And nothing happened. I did ask my mother and she told me something vague about relaying a call and that the button did not work on our line. She didn’t really know what it was herself. Nobody did. Not then and not now.

And yet most of the telephones from our (Dutch) PTT did have such a white button. And not only those of our PTT, but also those in other, mostly European countries, had a similar white button. Perhaps in a different colour. But there was always a button. And many people wondered what it was for. They could be found on the good old T65, but also on German and Belgian telephones.  

The mystery explained

A black "white" button
A black “white” button

Finally here is the explanation: the white button is a so called earthing button. It only works on particular, private installations and office networks. It does not work on the public network.

When you pressed the button, whilst speaking with somebody on an outside line, the other party was put on hold. Then you could dial another extension and consult with somebody else. If you pressed the earthing button again, you were reconnected with the other party. If the earthing button on the other extension was pressed instead, the call with the other party was transferred to that other extension.

The technology

White button that is red
White button that is red

The earthing button connects the A wire with earth or ground. This only works when all equipment in the network has a common mass or ground. Because of this, this system is only suitable for smaller networks and not for networks with above ground lines.

The earthing button is not the same as hook flash

In North America hook flash (also spelled hookflash) was used. Hook flash briefly interrupts the connection (a quick off-hook/on-hook/off-hook). The hook flash button simulates this. This technology originated in the era of manual exchanges. Old movies often show somebody operating the hookswitch on a candlestick telephone yelling: “operator, operator!”.

And the buttons are gone again

White button with PTT logo
White button with PTT logo

Funnily enough many phones were once equipped with such a button. In the Netherlands from the late 60s until the early 80s all telephones were equipped with such a button, even though most of the times it could not be used.

On modern telephones these buttons have disappeared. With modern digital signalling technology such a button is no longer needed. And nobody knows anymore what it was actually for. Well, almost nobody.

Have you ever pressed such a button? Did you enjoy this article? Or do you have any questions? Please leave a message below. 🙂



  1. Geweldig! Dit zijn van die weetjes waar je in principe niet veel aan hebt, maar tegelijkertijd dus wel. Want hiermee is meteen het mysterie ontrafeld (zelf ook vaak op gedrukt zonder resultaat)

  2. thanks for the explanation Arwin. based on your article, may I infere all (almost all) European telephones of that time were suitable for home and office use, existing not office-dedicated equipment?

  3. Hoi Arwin, dat ik er na meer dan 45 jaar achter kom waar het witte knopje voor is….te gek…maar leuk om te weten.
    Leuke site overigens, bij toeval tegengekomen, omdat ik met een wonderfoon bezig ben en info nodig heb over een scharnierpunt welke ontbreekt in een bakeliet toestel waar ik aan werk.
    Gr, Alex.

  4. “Ik had het wel eens aan mijn moeder gevraagd en zij vertelde iets vaags over doorverbinden en dat het knopje niet werkte op onze aansluiting”.

    Ha, ha. Ik zou bijna denken dat we familie zijn, maar waarschijnlijk had in de jaren 70 iedereen zo’n moeder. Heel herkenbaar voor mij in ieder geval!

    En een duidelijke uitleg.

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