Billing is one of those problems that seem deceivingly easy. Just talk to someone who has worked in telco billing and learn about the 4 horsemen of billpocalypse: metering, mediation, accounting, billing.
Rating can be great fun. US taxes (geocoding, urgh) are an absolute nightmare. Reversed charged calls, revenue sharing. Roaming is a pain (ASN.1 files). Even document numbering is a nightmare in some countries, as are legalities surrounding incorrect bills (corrective invoices) and crediting. Did 15 years in billing (now do finance software), and whenever I thought I knew it all there would be another curve ball thrown.
You can add taxes and negotiated contracts to the list tbh