This is the message on OCS:
Your monthly bill statement will be viewable online after 8 working days of your billing date.
8 days seriously? Are we still in year 1998?
I once asked my friend who works in a telco the same question. I remember some points that he said:
1) to generate millions of bills, it's not something that can complete in a short time
2) before generating the bills, finalizing the charges also needs time.
3) especially those with roaming charges, I used to get roaming charge 2 months after I came back from overseas. This is because some oversea charges are not real time - overseas telco needs to charge your home telco and your home telco will charge you in your bill - all these consume time also. which is why last time the so-called "credit limit" won't work with roaming charges - will go up to thousands coz overseas telco wouldn't know how much is your credit limit. luckily nowadays the telcos are offering maximum per day rate for roaming.