I worked on billing for a small UK telco about 18 years ago, and mostly it came down to reasonable-sounding business logic actually being a complete shitemare t

I worked on billing for a small UK telco about 18 years ago, and mostly it came down to reasonable-sounding business logic actually being a complete shitemare to make sense of. We got in Call Data Records (CDRs) every day from BT, which we then had to calculate billing for based on our business rules, which had to be somewhat aligned with BT's pricing so we'd actually not lose our shirts. It was pretty straightforward with one price between 8am and 1pm, then a cheaper one between 1pm and 6pm, then calls were free from 6pm until 10pm as long as they were less than one hour then billed at an even lower rate, with an hour of free any time minutes that reset at midnight. So, if you never made more than one hour total of calls between 8am and 6pm, and kept your calls to less than one hour the rest of the time, the actual call were free, right? You would genuinely be surprised how many times the billing system they were using would toss its cookies because someone made a phone call at 1740 (billable), continued right through 1800 (not billable) to 1900 (billable after 1 hour but there's still 40 minutes on "anytime" left so logged but free), right past 1940 (billable again) and on past midnight (free for another hour until anytime runs out). No, you can't just kick someone's call off after an hour.