It's not a telephone. (I think I just fell into the trap but I'll carry on....) It's the War Department "Broad Arrow" symbol used by the WD from the 17th century onwards (?) to indicate military equipment or sometimes locations used for surveying. The big question is why it needs one when it has a dirty great machine gun on it! Royal Enfield built some of these things (outfits, not the weapon) too but I don't know if any survive.