Wendelspanswick
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Pin 4 on a telephone socket is for Earth Recall, used on on PBX systems to sieze a new line while keeping the original caller on hold, and for Earth Calling lines, a line that will not give you dial tone unless it is briefly earthed, designed to stop you dialling out at the same time as a call was coming in.
Nearly all redundant now as phones use Timed Break Recall instead.
There is no advantage to using 4pr in the home, 3 or even 2 pr will do. We use 4pr on site as it means if an office wants an extra extension we can turn a single socket into a double, or even 2 doubles just by changing the face plates.
Nearly all redundant now as phones use Timed Break Recall instead.
There is no advantage to using 4pr in the home, 3 or even 2 pr will do. We use 4pr on site as it means if an office wants an extra extension we can turn a single socket into a double, or even 2 doubles just by changing the face plates.




that was over 32 years ago
So you should re-use the old one but if you want to take a chance and upgrade make sure it is one with the bt openreach logo on it. The best type of one to fit is the nte5 with a filtered faceplate as that does away with all the filter dongles at each phone socket. The disadvantage is that your router has to be located at the master socket. 