Reading CDRs: what your call records tell you about route quality

Call Detail Records are the transaction log of your voice network. Every call that completes — or fails to complete — generates a record that captures timing, routing, and outcome data. Most operators collect CDRs primarily for billing purposes, but the same data contains significant operational intelligence that can identify route problems, detect fraud, and support compliance reporting before issues become customer complaints or regulatory incidents.

What a CDR contains

A standard CDR captures the originating and terminating numbers, the date and time the call was initiated, the time at which the call was answered (if it was answered), the call duration, the carrier or route used, the SIP response code for calls that did not complete, and in STIR/SHAKEN environments, the attestation level of the call. More detailed records may include the originating IP address, the codec negotiated, and packet loss statistics for the media path.

The combination of these fields allows meaningful analysis of call quality, completion rates, and routing behaviour — but only if the data is being collected consistently and reviewed regularly rather than stored and ignored.

Answer-seizure ratio

The answer-seizure ratio (ASR) is the percentage of call attempts that result in an answered call, expressed as a proportion of all call attempts on a given route or to a given destination. An ASR of 70% means that 70 in every 100 calls to that destination are answered. The remaining 30 are failing at some point in the call path — either before ringing, during ringing, or due to busy signal.

ASR varies significantly by destination type: calls to mobile numbers typically have lower ASR than calls to geographic numbers, international destinations vary widely, and calls to toll-free numbers have different characteristics again. Establishing a baseline ASR for each destination category you carry, then monitoring for significant deviations, allows you to detect route degradation before it generates customer complaints.

A sudden drop in ASR on a route that was previously performing normally is a strong signal of a routing or termination problem. A gradual decline may indicate a carrier-level issue or a change in the destination carrier's network that requires route adjustment.

Post-dial delay

Post-dial delay (PDD) is the time between a call being initiated and the called party's phone beginning to ring. In IP voice networks, PDD is affected by SIP signalling latency, codec negotiation, route selection, and any intermediate processing between the originating and terminating carrier.

PDD above approximately four seconds is noticeable to callers. PDD above eight seconds is typically experienced as a fault, and many callers will abandon the call before it rings. CDR data that captures the timestamp of the SIP INVITE and the timestamp of the 180 Ringing response allows PDD to be calculated and trended over time. Routes with consistently high PDD are candidates for replacement or reweighting in your routing table.

SIP response code analysis

Calls that do not complete generate SIP response codes that describe the reason for the failure. These codes are highly informative once you know what to look for. A high volume of 404 responses (not found) on a specific destination suggests number range issues or porting problems. A high volume of 503 responses (service unavailable) suggests a congestion or capacity problem at the terminating carrier. A high volume of 486 responses (busy here) may indicate legitimate busy conditions or, in high volumes, may point to a destination that is not accepting traffic correctly.

Grouping CDR data by SIP response code for failed calls, broken down by destination prefix or carrier route, gives a quick diagnostic picture of where failures are concentrated. This analysis is particularly useful after a network change or during a suspected service degradation event.

CLI delivery and stripping

CDR analysis can reveal whether calling line identity is being delivered correctly to terminating destinations. If CDRs show that calls are completing but downstream carriers or customers are reporting that CLI is not being presented, the CDR data for affected calls — specifically the FROM and PAI headers captured at your switching point — allows you to determine whether CLI is present when calls leave your network, or whether the problem is introduced at an intermediate carrier.

CLI stripping is a particular concern for calls crossing international boundaries, where intermediate carriers may modify or remove CLI during transit. Regular spot-check analysis of CLI delivery, using test calls to numbers where you can verify presentation, should be part of your quality assurance process.

Using CDRs for compliance reporting

CDR data is the primary evidentiary record for several compliance obligations. STIR/SHAKEN attestation levels are captured in CDRs and can be reviewed to confirm that attestation assignments are consistent with your obligations — for example, that A-level attestation is not being applied to calls where you have no customer relationship with the originating party. DNCR and TPS compliance monitoring benefits from CDR analysis that cross-references called numbers against register data retrospectively, as a sanity check on real-time filtering. And in the event of a regulatory inquiry or traceback request, CDR data is typically the primary source from which you reconstruct the call path.