They "solved" it so: each morning one person imported the data from both systems into huge excel spreadsheet (file had almost 100 MB), then she send the file to person that know how to correct all the duplicated and missing data in a few columns (it wasn't turing-complete process, it was heuristic, cause systems used different subdivisions and codes for same things, some data required to be sure how to integrate them was missing, and some data was changing without reflection in the systems - like the ingredients used to make one unit of product could change every day, and NOBODY KEPT RECORD of when and how it changed, only some people in production knew how they make the product today).
Then that person send the file to the next person, that knew how to fill the next few details. And so on, close to 15:00 file was supposed to be ready to send to the management. Sometimes it took till 20:00 (hours after 16:00 wasn't paid). When somebody important was on leave, nobody knew how to fill the details.
That spreadsheets had almost no formulas in them, cause people "cut and pasted as values" all data after calculating it (my wife was told it's to "protect the data from accidential changes"). I think it was to prevent people from seeing the errors.
Everybody knew that the data is all wrong. Nobody knew how to even get the correct input for most fields, it was all fuzzy (for example - one person in production knew which labels are bought in 100s, 500s, 1000s, etc. He was sent the file to fill that details, nobody could check that, and there was always some contradictions in the last pages of the spreadsheet. Most of the work went into massaging it untill it seemed OK. The file also crashed half the time, even on computers with 4GB of ram and 64bit windows.
I wonder, how they managed to stay in business (they are still around, and have quite big marketshare).
Verizon's billing system is the same way. Is it bad? Yes. Is it worse than AT&T's? Probably not. Are people going to quit Verizon because of billing errors? Well, I did, but most people wouldn't.