Two or sometimes four-wire. Four wires are used to prevent eavesdropping. (If you have picked up one phone, the other phones on the line are "dead")
The plastic pin has to do with this too. When inserted it loops the feed-through via the phone (or other equipment) inserted. Imported two-wire modems and phones give you problems when you want both phone and modem on same socket. Phones are not my cup of tea, really. Therefore I doubt I can answer any questions.
As you can see, they accept RJ-11 plugs. However, the trusted old "Rikstelefon"-sockets remain the standard in new construction, despite serving only as RJ-11 adapters.
For some reason RJ-45 has now caught on as phone plug. I installed RJ-45 sockets for this use a few years ago, when they were not normally used for this purpose. Today you can buy RJ-45 extension reels.
[This message has been edited by C-H (edited 11-03-2002).]