Friendface - S3-E5
Other mistake: In the beginning of the episode, Roy is explaining why Friendface is bad and chastising Jen for being on it. If you look at his monitor, he is on a website with a green color palette like the Friendface infomercial and a layout similar to MySpace. While I can't be 100% sure, it seems like a graphic designer created a mock website of Friendface to be used in the show. If true, that means Roy is already on Friendface before Jen even enters the room. (00:01:00)
Revealing mistake: When Moss runs into the door, all walls on the set shake. (00:06:25)
Continuity mistake: Season 3 Episode 1 'From Hell' When Roy says "Nice glasses" and Moss replies with "Not as nice as your momma's glasses", The Red Door between Roy changes from being closed to being open.
Answer: It's a joke on the "118" numbers in the UK. 118 numbers were used for directory enquiries and adverts for them on TV always tried to give a way to easily remember one for a particular company. The fake ad in the show takes it to the extreme. People in the UK do not use "911", the point of the joke was that Roy had forgotten "999" while saying it was easy to remember.