It seems that once the currently exclusive to EE is over with and done you can count on Three UK to stock the Nokia Lumia 920. This was widely thought to be the case but was confirmed by the tweet you see above you yesterday. Vodafone is another carrier expected to carry the Lumia 920 once the green light is given. I can’t find any information on what the exclusivity period is I believe it be six months but I can’t confirm.
