On this occasion it's not number unknown, it's my letting agents. They could be calling about anything, perhaps a leaking pipe in the flat upstairs, perhaps a tedious document that needs signing, whatever. I slip away from my desk and answer.
"Hi it's Jason," says apparently-Jason. "Your landlord would like to do a valuation on the flat. It might be for the mortgage or something."
There is a pause while we both let the implications of that last little white lie sink in.
"Can I make an appointment to come over? How about tomorrow evening?"
When tomorrow evening arrives I dash home on the early side to make sure I'm properly prepared for Jason's arrival. The washing up is completed, a pile of trousers is relocated and some strategic dusting is undertaken.
The door buzzer goes as dusk is falling, and Jason makes his way up to my flat. He's young and dapper, maybe half my age, with a clipboard stashed underneath the arm of his jacket.
"Do I need to take my shoes off?"
Jason's brogues stay on his feet as I point him in the general direction of the first couple of rooms. A waft of something aftershave-y lingers in the hallway.
"The flat's bigger than it looks, " I venture, before realising I probably shouldn't be overselling the place, not in the circumstances. Too late, he's probably already converting my available cubic metres into equivalent property value.
"And is this the master bedroom?"
It's been a long time since any visitor to the flat asked that question. In this case the answer's no, it's bedroom number two, which again I realise is a black mark against the affordability of my rent.
He asks me whether I've been living here long, which compared to everyone else in the building I have. Indeed during that time I've paid my landlord a six figure sum, which ought to make me sob, but that's the reality of London life.
I briefly wish the lounge looked smaller as Jason gives it a brief once over.
"Well I think that's everything," he says, and within ten seconds we're shaking hands with pleasantries at the door.
It's possible that nothing happens next, that my landlord really did only want to know how much his buy-to-let investment is worth. But it's also possible that an email will arrive in the immediate future wondering whether maybe, possibly, thank you, I could stretch to another hundred or two a month.
Bow's gone up a bit in the world since the Olympics, somewhat beyond anything I might have expected when I moved in. Indeed I'll soon have near neighbours from Malaysia who've never even set foot in East London, let alone their new penthouse flats overlooking the roundabout. Jason, I suspect, still lives rather further out.
So I'm not expecting anything awkward to transpire following his visit, to become one of those edged out of a neighbourhood by property's rising tide. But I will be giving my inbox a sideways glance over the next week, just in case an unexpected demand fires in. In London these days it costs more simply to stand still.