The American modular plug has become the de facto world standard, but there are complications, as not everywhere which has adopted it wires the jacks the same way. Ireland uses it now, but the line is always wired on the outside pair (what would be the yellow/black pair in the U.S.) and they use one side of the inner pair as a bell feed in the same manner as U.K. wiring.
Although in general I like American NEMA power connectors, I have to side with Sven on this one and say that I don't like the modular telephone jacks at all. The plastic clips break off too easily, and they're not at all easy to use for anyone with limited dexterity in their fingers. The old 4-prong jacks were far superior in my opinion (BJ will know from another forum that I've just recently been searching for a source of these!)
Another point: The modular jacks and plugs are used at the telephone end of the cord as well. That means that if one end of a cord is connected to a phone line there will be exposed voltages (up to 50V DC and 90V+ AC ringing) on the other end, easily touched.
Although the U.K. has its own design of modular jacks (just as bad if not worse than the U.S. types, I have to add), we are seeing the U.S. modular plug at the phone end used here as well now. In some cases, the latch clip is shortened so that the "live" end can only be removed by using some sort of tool (e.g. a very fine-bladed screwdriver).
Trumpy,
Strictly speaking the RJ designations refer not to the plugs/jacks themselves, but rather to the way in which they are wired. The RJ means Registered Jack, and the numbers were standardized by AT&T/Bell under their USOC system (Uniform Service Order Code).
RJ-11 just happens to have stuck as the name because this is the wiring arrangement for a standard single-line jack as used in most domestic installations.
The same plugs/jacks can be used for other arrangements. For example, a two-line system (line #1 on red/green, #2 on black/yellow) is properly called RJ-14.
