When shall we three meet again? Whitehall, the House, or Number 10?

[Poll #1845875]

ETA: Bonus points for anyone who can spot the reference!

AKICOLJ: xkcd

I'm trying to find an xkcd cartoon. Someone is locked out, and is trying a number of technologically-advanced and esoteric ways of getting in touch with their flatmate; but hasn't thought to ring the doorbell. Any ideas?

ETA: Now solved, by the estimable ladyofastolat.