Best slot type for collecting an account number in a Lex bot

I'm working on a Lex bot to accept a caller's spoken account number (9-12 digits) and 4 digit PIN. And while Slot type AMAZON.FOUR_DIGIT_NUMBER works great for the PIN, I'm running into difficulties with the account number. Slot type AMAZON.NUMBER sometimes works. But I've also been cut off in the middle of speaking the account number. For example, it will sometimes cut me off after speaking only 5 numbers - and it's not because I'm pausing or anything.
Is there any way to specify that it should listen for at least 9 digits, and no more then 12?