Earth Recall & flash

[mR_Slug]

I've seen a number of phones originally used on PBXs. As I understand this is equivalent to or related to the US term flash. Recall in this context means recall the operator/PBX for additional functions like call transfer. There's this definition https://www.britishtelephones.com/glossary.htm#EARTH%20RECALL, Just curious about the history of this. There's quite alot of GPO 300 series phones with this feature, that ended production in the mid '50s. So we have at least the '50s when this was introduced, but was it used with manual exchanges too? When did it start/end? Not much I can find about it.

Curious If anyone could give a bit more detail. Has anyone got these buttons working as originally intended?

[countryman]

Earth keys were common throughout Continental Europe at least from the 1930ies on - e.g. you frequently find German Model 28 with them. Their use ended with the broad introduction of touch tone dialing during the 1980ies. The purpose was to trigger a function of the PABX the phone was hooked to. The link you gave mentions Earth Recall vs. Timed Break Recall (flash signal).
The actual function was depending on the exact setup of the PABX. Mostly it had to do with the transfer of the call to another extension, or it might put a call on hold while doing another call with the same phone. Also, the key might toggle between internal and external calls.

German W48 usually have what is called "Earth and Flash key" (Erd- und Flackertaste). Setting a jumper selects if the a-b loop opens or remains closed while line a is connected to earth. Depending on the PABX one or the other setting is required. Opening the loop without earth connected allows to use this button as a "modern" flash key, provided the PABX is tolerant with the flash time. On my VOIP equipment I can toggle between internal and external calls this way.
The literature also mentions flashing (flackern) on manual PBX to call the attention of an operator, but I'm not sure how common this was.
Remaining PABX actually using the earth key in the originial function must be VERY few, and in the hand of serious collectors or rather museums.

[countryman]

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[tubaman]

Where I work we had phones using the earth recall facility until only a couple of years ago so I wouldn't be at all surprised if there are still places using them today. As @countryman has said it was used for call transfer and hold type facilities. We could also set up things like 'transfer busy' and 'transfer no reply' using key sequences that involved the recall key. 

[dsk]

The last version of phones with Earth recall in Norway was introduced in 1982, so my guess is that such exchanges was sold here to about 1990. When flash was introduced here in Norway the timing was electronic and slightly longer than a single dial pulse.
Hook flash was never a standard here.

[countryman]

The short flash is the culprit of all the trouble with rotary dial phones on modern electronic equipment. Even if the design is pulse capable, it is intended to recognize a 80 ms flash as well. A dial pulse (depending on the local standard) would be 50...70(?) milliseconds. When the dial is just a tad slow on any of the pulses it sends, a flash signal will be recognized, even if the average speed of the dial is correct.
A flash of a well determined length wasn't easy to generate before micro electronics. A longer flash time was used temporarily especially on some German made PABX but it easily caused "hiccup" when a user hung up only briefly before the next call. Flash, overall, wasn't compatible very well with pulse dialing, that's why earth recall was preferred although it accounted for a third wire in a PABX system.

[mR_Slug]

Thanks for explaining this. I was planning on a 2 wire system for my phones, but a 4 wire system may allow for some additional functions. Quite how this could be used in a modern/voip setup, I don't know. I'm curious about building a 'hybrid' system. Some old and new tech combined. It would be nice to preserve a vintage PBX, but '50s tech is bulky, so perhaps bits of it.