Also, for DIY-friendilenss IDC connectors arn't used for the extension connection on the back of the plate. We just have a pair of screw down connectors for each line.
That's a interesting point. Given that the whole intent of the NTE5 was to allow for DIY extension wiring without anyone having to get at the network side of the interface, it would have probably been more appropriate to provide screw terminals. The extension jacks are availble in both IDC and screw-terminal versions.
Many DIYers will just try to jam wires into IDC connectors with any convenient small screwdriver. I've had a lot of bad-connection reports which have turned out to be IDC connectors which have obviously been so abused.
The designation of the terminals here is :
L1 (tip)
L2 (ring)
R - obsolete ringer wire.
The BT jack coding is just numbers: 2, 3, 4, 5. Assignments for a standrad single line are:
#2 = Ring
#3 = Ringer wire
#4 = Spare
#5 = Tip
Jacks also have 1 and 6 for the outermost pair, but they are not used on normal domestic wiring and even if 3-pair cable is used, they are often left unconnected, the white-green pair just coiled up loose in the box.
As I've mentioned before though, thanks to a monumental screw-up in communications when the new BT connectors were being implemented in the early 1980s, the
plug numbers run the opposite way to those on the jack (i.e. 1=6, 2=5, 3=4).
I've seen plenty of cheap jacks in which the manufacturer has erroneously used the
plug numbering for the
socket. You can dial out, answer calls, as normal of course as you just have tip/ring reversed, but because the ringer wire ends up on #4 instead of #3, there's no incoming ringing. That misnumbering causes a lot of confusion when people links the numbers and don't realize what's happened.
If you'd multiple extensions then anti-tinkle wiring was done if necessary. It depended on the phone models etc etc. The Nortel 500 style phones didn't appear to tinkle anyway.
Western Electric 500 (and 1500/2500) don;t need it either. The North American phones don't seem to suffer from the "dial tinkle" problem due to the different ringer design.
The old British GPO 300 and 700-type phones with unbiased ringers did though, so the 3rd wire was standard provision where extensions were involved. For party line service, they also included extra wiring to prevent the station Y phone from tapping when station X dialed, and vice versa.
On the 700-sets, a small thermistor was fitted in series with the bell. On earlier 300-type wiring, they would sometimes actually use a bridge-rectifier and slugged slow-to-operate relay to connect the bell to the line only when incming ringing was detected.