What kind of socket is this?
Yes, indeed it is a Belgian phone socket. 

http://www.levoyageur.net/phone-country-Belgium.html

The four pins match the four holes in your socket. The fifth pin is there to make sure you don't put it in upside down. There's a hole for it too.

This is how it looks with a cover (except for the RJ jack that you may not have)

https://web.archive.org/web/20140716003426/http://www.p-wholesale.com/cn-pro/5/121to2/phone-jack-91449.html

Below I mark the five pins and the switch at the center. The switch is opened by the b and S pins indicating that the socket is used. This is common for phone sockets.