I found it in my apartment's wall.
Yes, indeed it is a Belgian phone socket.
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)
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.
I’m not familiar with outlets in Belgium, but assuming they are typical European outlets, then it looks to be a high-voltage stove/oven or dryer electrical outlet (why it would be in the living room though is beyond me).
The labels for the four connectors (
a, b, S, ground) are curious, but presumably they are hot (a), hot (b), neutral (S), and ground:
There are also two other connectors (
c, d here) which are specifically part of a stove/oven outlet as opposed to a dryer outlet: