A Caller Line Identifier, commonly referred to as a CLI, is a unique identifier the provider/carrier will associate to a specific customer on the Call Detail Record (CDR). This could be in the form of a phone number, SIP Trunk, or custom account code identifier.
Within Billingbooth you assign these CLIs to your own customers, so when it's time to upload calls, Billingbooth knows which of your customers to assign call charges to. You can further expand this functionality by assigning CLIs to a specific cost center, enabling you to target specific locations, offices or departments etc. of your customer.