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If you include 0000, ten thousand unique four digit codes are possible.
There can be 103 = 1000 codes.
0,1,2 is small
It is -987. The smallest positive 3-digit integer with unique digits is 102.
There are 900 possible three-digit numbers not beginning with 0. (Note, however, that this question does not accurately describe the restrictions on numbers that can be used as area codes.)