When in doubt, put it to the t-test.
I only have one part: other than zero, it's the first nine digits in alphabetical order. It's also divisible by nine.
Good job! That's actually both parts (the first person I ran it by said "it's all nine digits" so I meant to elaborate that it was more than that. The alphabetical part is key.I love this problem because you really have to switch parts of your brain to get it. First numbers as digits, then numbers as words.