A standard tetrapolar telephone plug has four round metal pins and one plastic pin. The design is only used in Belgium

Belgium

Belgium , officially the Kingdom of Belgium, is a federal state in Western Europe. It is a founding member of the European Union and hosts the EU's headquarters, and those of several other major international organisations such as NATO.Belgium is also a member of, or affiliated to, many...

 for telephone wiring. It is similar to the tripolar telephone plug

Tripolar plug

The telefonic tripolar plug is the first type of telephone plug used in Italy.-Structure:It uses three pins: the two on the top are joined with the twisted pair of cables bringing the signal; the third, off-center, can be used for loop through when more sockets exist.Every plug complies with...

 of Italy

Italy

Italy , officially the Italian Republic languages]] under the European Charter for Regional or Minority Languages. In each of these, Italy's official name is as follows:;;;;;;;;), is a unitary parliamentary republic in South-Central Europe. To the north it borders France, Switzerland, Austria and...

 and also the Swedish telephone plug

Swedish telephone plugs & sockets

A standard Swedish telephone plug carries one telephone line and has four flat metal pins and one plastic pin. The design is only used in Sweden and older installations in Iceland . Neither plug nor socket is compatible with other plugs and sockets...

.
The plastic pin adds a presence function. When not inserted into a jack, the jack itself (mechanically) connects the incoming line to the next socket.