customer troubleshooting process

I've been asked to look into customer service issues/fault resolution Along the lines of: Fault1: question1 question2 Fault2: question1 question2 eg Fault:Subscriber can't authenticate (and then get inetaccess) Q1. which userid/password was used Q2. which access number But, mmm, hasn't this been done many times before? I searched the web, but found some half useful stuff on troubleshooting. Can anyone point me to a relevant URL? TIA Tony