If it is a four-digit number, you might just systematically, or by sheer luck, plug it in. For a four-digit number there are only
10,000 combinations, 0000-9999. If all attempts to get the code fail, I would try plugging in 100 combinations a day, starting with 0000 and up on day one, 9999 and down on day two, 0100 and up on day 3, 9899 and down on day 4, and so on. You could also marathon it and plug in hundreds of them a day and try to hit it. I must add that I know nothing about the security on a 9900. If it goes into some other kind of lockdown after so many incorrect codes are entered, then this plan would not work, unless you are very lucky.