Nowadays, only the first digit means anything. The other three just grow as new models are added. 1xxx and 2xxx phones are basic models. 3xxx are "expressions." 5xxx are "active." 6xxx are "classic," 7xxx are fashion phones, 8xxx are "premium" and 9xxx are Communicators. (One thing to note: our prototype 7280 wasn't in the same colors as the final product, which will have a red, rather than green light on the end and a white, rather than red fabric tag hanging off the side.) There are no 4xxx models because the number 4 is considered unlucky in parts of Asia.
source: BNC