One of my earliest jobs was working for a mobile-phone billing company, in Scotland.

Every day we'd use UUCP to copy billing-records from random-systems to a central host, running SCO Unixware. Then the daily "billing run" would total, reconcile, and massage the data.

The result of the processing would be a pile of CSV files which would then get further massaged by a collection of Perl scripts.

Days I definitely do not miss!

I did some consulting for a Telco on their billing run. It ran on a very, very large computer for its day. The run typically took 16+ hours, sometimes as many as 22 hours, leading to some very white knuckle moments when it threatened to delay the start of the next day's run.

For every telecom company I worked for, billing was a mess.