Written from the POV of a former BT billing systems developer - The system was designed (fucking badly) before the widespread existence of ADSL.

This is what happens when you outsource core financial systems to low cost bidders with dubious tech chops building a message queue system is not fraking rocket science at this point.

Back when I worked on the ground up billing system for Telecom Gold (aka Dialcom) we did this as the existing mish mash of dodgy code that Dialcom offered (Sorry Eric) was not up to standard.

We had large amounts of internal auditing built in and we tracked discrepancies to the Penny.