Yep thats a phone socket.
Locate the "omnibox" (usually brown) fixed to the external wall and trace where the cable from it comes into your property. The socket is usually pretty close to that on the inside wall.
If its not evident it may have been removed or covered over and you may need a "tech install".
Or is it a "flat" that is fed from an internal box?
--------------------
John
--------------------
I do not work for VM. My services: HD TV on VIP (+ Sky Sports & Movies & BT sport), x3 V6 boxes (1 wired 2 WiFi,) SH2 in modem mode with Airport Extreme Router +2 Airport Express's. On VIVID200, Talk Anytime Phone, x2 Mobile SIM only iPhones.