A call detail record is the single most useful artefact a gateway produces. It is how you bill, how you reconcile with carriers, and how you diagnose problems. Reading CDRs well is a core operational skill.
The fields that matter
- Start, answer and end time: the gap between start and answer is ringing time and should never be billed as talk time.
- Gateway, device and SIM: which physical line carried the call.
- Source and destination: who called whom, and which route matched.
- Duration and billable duration: billable time may be rounded by your rating rules.
- Rate and cost: what the call cost under the applicable package.
- Status and disconnect cause: why the call ended.
Disconnect causes
Every failed or ended call carries a cause. Normal clearing means one side hung up. User busy and no answer are ordinary outcomes. Causes such as network congestion, unallocated number or temporary failure point to problems worth investigating, especially when they cluster on one device or one carrier.
On phone-based gateways, the most precise cause often comes from the handset itself. A platform that captures the device-side reason alongside the SIP response gives you a much clearer picture than SIP codes alone.
ASR and ACD
Answer-seizure ratio (ASR) is the share of attempts that were answered. Average call duration (ACD) is the mean talk time of answered calls. Together they describe traffic quality.
- A sudden drop in ASR on one device often means a SIM or signal problem.
- A drop across a whole site suggests a network or carrier issue.
- Very short ACD on a line may indicate calls being answered by an announcement rather than a person.
Making CDRs useful
Filter by site, device, route and time range to isolate a problem. Export to CSV or Excel for reconciliation. Compare the same metrics week on week so that you know what normal looks like.
It also helps to agree internally on which duration is the source of truth. Carriers, your SIP platform and the gateway may each measure a call slightly differently. Documenting which figure you bill from, and how rounding is applied, avoids long disputes later.
Finally, retain CDRs for as long as your billing disputes, contracts and local regulations require — and no longer than your privacy policy allows.