Hi there, the advanced payment would not be applied on the first month but on the last month of your contract. Keep in mind that there's also a termination fee of 550 (?) that will be deducted from your advanced payment when you cut your line on the 24th month as per the CSR I spoke with.
As for your due date on August 12, I think you should be billed for a pro-rated amount from July 10 to July 17 PLUS 999 to cover July 18 to August 17. Which means that if I am right, your bill will be more than 999. I think that is the case because I was studying my billing statements from Globe and noticed that my payments are in advance apart from the advanced payment I made when I applied for my phone line. But my advice is for you to call their hotline (211 then press 1, 1, 11-digit mobile number, 3, 2) so that they can explain it to you further since I might be wrong. Better yet, wait for your bill and see how the breakdown looks like.