Billing CDR Technical

CDR-Based Billing Explained: From Phone Call to Invoice Line

By BlueRockTEL Team

What is a CDR?

A Call Detail Record (CDR) is a data file generated by a telecom switch or SIP server that captures the details of a single communication event. A typical VoIP CDR contains:

  • Calling number (A-party)
  • Called number (B-party)
  • Start time and end time
  • Duration (in seconds)
  • Routing information (which carrier handled the call)
  • Call result (answered, busy, no answer)

For an operator serving 500 business customers, each making dozens of calls per day, this adds up to millions of CDRs per month.

The Rating Process

Rating is the process of assigning a monetary value to each CDR. It involves three steps:

1. Normalisation

Raw CDRs arrive in different formats depending on the switch vendor. The billing engine normalises them into a standard internal format before processing.

2. Destination Classification

The called number is matched against a destination table. Is this a national call? Mobile? International? Premium rate? The answer determines which rate applies.

3. Rate Application

The rate for the destination is applied to the call duration. This might be:

  • A flat per-call fee
  • A per-minute rate (rounded to the nearest second, 6 seconds, or 60 seconds depending on your contract)
  • A tiered rate (first 100 minutes free, then €0.002/min)

The output is a rated CDR: the original CDR plus a calculated cost.

From Rated CDRs to an Invoice

Once all CDRs for a billing period are rated, they are aggregated by customer account. The billing engine:

  1. Groups calls by customer, by destination category, by service line
  2. Adds recurring charges (monthly subscription fees, hardware rental, etc.)
  3. Applies discounts or commitments (volume discounts, promotional pricing)
  4. Generates the invoice as a structured document with line items per service

In BlueRockTEL, this entire process runs automatically on a schedule. For an operator with 500 customers, the full billing run completes in minutes.

Margin Analysis

A well-designed billing system does not just generate customer invoices. It also tracks the cost side:

  • What did the carrier charge us for each call?
  • What are we charging the customer?
  • What is the margin per destination? Per customer? Per month?

This real-time margin visibility is one of the most valuable features of an integrated BSS — it allows operators to identify unprofitable customer relationships or pricing anomalies before they become serious problems.

Conclusion

CDR-based billing is technically complex but operationally critical. Getting it right — at scale, with accuracy, every month — is the difference between a profitable telecom operation and one that is constantly firefighting billing disputes. Purpose-built tools, designed for this specific problem, deliver reliability that generic accounting software simply cannot match.

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