When a member has an 084 call active, or other NGN with unpredictable charging rates, the airtime credit balance can drop quickly, with no warning till the call terminates unexpectedly all of a sudden. That can be incredibly problematic if one is "on hold" and has wasted the credit getting nowhere.
A text-message is sent at that point, when the credit hits zero.
This is *not* a general-purpose alert about a low credit balance... it's specifically relevant when a member has a call active which falls into the group of numbers for which rates can vary wildly. Premium rate numbers, 084 services, and so forth.
It should be simple enough to send a text-message when the airtime credit balance has dropped to approximately 5-10 minutes' worth of call duration. The system already tracks the balance, the charging is already being applied by the billing system, the state of the call being active is part of this process, the rate is clearly known by the system doing the charging, and in short, it should be a straightforward matter for the software when updating the balance information to also do one extra thing -- if it's an 084/NGN/etc. number and the updated balance is less than, say, ten minutes at the current rate being billed, then trigger a text alert.
The software that does the billing updates is already periodically making this check, with the data it needs to detect the above, there's nothing onerously more complicated to add from a functionality point of view.