I worked in IT for a Telco, and was intimately involved in the billing system, the multi-million dollar project to move everything to a new billing system, and then the new billing system.
From experience I can tell you the billing system of a Telco is at least 1000 times more complicated than I ever would have believed. Maybe 100,000 times. The number of edge cases and complicated stuff it has to deal with is utterly staggering, and every single one of them is being changed repeatedly by marketing and project managers and people from all across the company - products for partial months, discounts, packages, rate changes, non-recurring charges, changes to non-recurring charges, fees, taxes, government subsidies, overages, products that have to exist by law but we don't want to sell, products that customers have because "grandfathered", products that don't exist in real life but do in the billing system, etc. etc.
Keep in mind the billing system is extremely tightly coupled with the provisioning system (the one that actually makes the internet lights blink and the voices actually work), which has ten thousand more edge cases and complicated configuration than the billing system.
Once you dive into the provisioning system, the billing system looks straightforward!
My personal bill from the company I worked for often had mistakes, and frankly I was shocked it didn't have more!
From experience I can tell you the billing system of a Telco is at least 1000 times more complicated than I ever would have believed. Maybe 100,000 times. The number of edge cases and complicated stuff it has to deal with is utterly staggering, and every single one of them is being changed repeatedly by marketing and project managers and people from all across the company - products for partial months, discounts, packages, rate changes, non-recurring charges, changes to non-recurring charges, fees, taxes, government subsidies, overages, products that have to exist by law but we don't want to sell, products that customers have because "grandfathered", products that don't exist in real life but do in the billing system, etc. etc.
Keep in mind the billing system is extremely tightly coupled with the provisioning system (the one that actually makes the internet lights blink and the voices actually work), which has ten thousand more edge cases and complicated configuration than the billing system.
Once you dive into the provisioning system, the billing system looks straightforward!
My personal bill from the company I worked for often had mistakes, and frankly I was shocked it didn't have more!