I started and ran a small early (UK) ISP in the 90s! I can't remember how we did the billing but it was better than that. I suspect that it involved awk with : separated fields, because I remember my dyslexic colleague saying that all he could see was swimming : characters... And I (still) like awk.

I had more incoming phone lines than the rest of the street sometimes (this business was run from home), at several addresses around London as we moved. And we broke each telco's billing system in a new and interesting way...