This may also be related to roaming, but here’s where you can find out more.
It seems that you already know what CDR means, because you tagged your question with ‘Call Detail Record’.
What exactly do you need to know?
Actually CDR stands for Call Detail Record, and it is a record containing information relating to a single call or session. The format of CDRs is not fixed and therefore varies between networks. CDR are generated by telephone provider switches.
A CDR usually contains information like Call Setup time, Call Answer Time, Call End Time, Originating Number, Destination Number, involved protocols, etc.