Interesting. I’ve never seen a phone like that. Usually the wall mounted versions of the Model 500 had a hook for the handset in the front of the base to hang the handset vertically. This one looks like a different company than Western Electric though. I’m guessing it’s a UK company, because it’s 999 for emergencies (or at least it’s not US). You’ve got me curious enough I feel like I’m about to go down a rabbit hole.
Edit: yep, it’s a UK company called GPO. This is their model 741: