Originally, with the switch-over to subscriber provided phones, it was to prevent the simple use of non-approved US phones that sounded very quiet on the UK's +7/-16dB network. It was intended to avoid fault calls when the subs. couldn't hear the remote end when using a US and most other country sourced phones.
In the EU, approval testing is all done to ETSI specs nowadays. Individual country specs went about 20 years ago.
Back in the days of BT approvals (even before BABT), I did a number of trips to Baynard House to get modems BT approved. It was mostly about the safety of the product and any possible interference with the rest of the network, and they didn't really care if the appliance worked or not. If you go back a bit further (before my involvement), they used to care about products actually working, but I think they eventually decided that was the consumers problem, not theirs.
Some parts such as working out what the REN rating was were remarkably crude.
An RJ11 has two wires. There isn't an RJ11 with 4 wires. There's a 6P4C connecter, but it's not an RJ11. In telephony, the 6P4C is usually an RJ14, and carries two 2-wire phone circuits (and it's compatible with RJ11 for accessing just the first circuit). Again, if we'd used the 6P4C, we'd have something which looked like a regular RJ14, plugged into RJ14 and RJ11 sockets, but didn't work.
That's what I'm saying. They didn't all need 4 wires, but the circuit defines 4 wires. In some cases 3, or all 4 are used. Most phones nowadays only need 2 wires because they don't have real bells and use loop disconnect dialing, and don't have earth recall, but this wasn't the case when the connector was developed.
Ok, thanks. Since asking the above I've poked around a bit so its clearer now. Shame though.
Mind you, this shows up a bit the divide between the telephony and networking worlds. In 25 years of computer networking, no one in a variety of different organisations I dealt with has ever called it any thing other than an RJ11, whether it had 2, 4, or 6 wires.
and even more so with RJ45, which only has 1 pair (and is also partly compatible with RJ11 and RJ14 for the first circuit) and is completely useless for ethernet, when they really mean 8C8P or more specifically, T568A or T568B ;-)
Mostly it doesn't much matter, but it does matter when you want to understand why BT couldn't have used the RJ11 back when they developed their own connector.
ITYM "Most phones nowadays [...] use DTMF dialing."
Thanks all for the answers - some interesting stuff in there, and has usefully allayed any fears I may have had.
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