There's strict regulations for this kind of system:
- once the costumer charges credit for the phone, it must be available for use in less then 3 minutes;
- when a costumer makes a call (or sends an sms, mms, uses the data network etc), the amount of credit available must be validated in less than half a second, no calls should be dropped because of inability of the system to validate credit;
- and there's the DW, which has to show graphics in real time (and I mean, if someone wants to know how many calls where issued in the last 30 minutes, this information must be available); - and many other stuff, I won't write it all down in a post at
Do you have any idea how many charges a day this specific telco receives? How many calls? The amount of transactions in the databases is at the very least one or two orders of magnitude higher than the amount twitter has to deal with.