ASR, ACD, PDD, NER and MOS: VoIP Quality Metrics Explained

Voice traffic may not always meet your expectations. What's stated in the specifications actually performs differently in real-world use. VoIP quality metrics are the bread and butter of getting value for money.

This isn't just theory and speculation. It's a difference expressed in concrete numbers. You can literally feel the difference between a profitable wholesale route and one that's irrelevant and doesn't generate the desired profit.

Our article on ASR, ACD, PDD, NER, MOS explained all the processes in practical terms. For SIPeasy Telecom, route monitoring and real-time VoIP monitoring have become a daily habit. By the end, you'll learn five metrics that will be useful for call center managers, resellers, or customers of similar services.

Point Number 1: Answer-Seizure Ratio as a Valuable Metric

ASR is the ratio of answered calls to total call attempts, expressed as a percentage. Sometimes called call completion rate or call success rate in carrier and CDR reports. 

For example, if you send 100 calls and 42 get picked up, your ASR is 42%. It's the first thing anyone glances at when evaluating VoIP performance metrics. Mostly because it's easy to calculate and impossible to ignore. So, ASR is one of the best KPIs for wholesale VoIP buyers to check before signing off on any route.

Point to mention: ASR alone tells an incomplete story. It doesn't distinguish between failed calls because the number was busy and failed calls because the carrier's route was broken. That nuance matters a lot once you start comparing suppliers.

What Is ASR in VoIP and How Is It Calculated?

To calculate ASR VoIP, you need to divide answered calls by total call attempts, then multiplying by 100 to get a percentage. It's the most-cited of all VoIP call quality metrics because it's simple to pull from any CDR export. This is really the starting point for anyone learning how to evaluate VoIP call quality properly.

What Is a Good ASR for VoIP Termination?

An ASR in the 40–60% range is often considered normal for wholesale termination. We at SIPeasy Telecom say that since a large share of unanswered calls comes from legitimate no-answers rather than technical failure. Anything below 30% usually signals route problems worth investigating. 

Point 2: Average Call Duration as One of the VoIP Quality Metrics

With ACD you can measure the average length of answered calls, usually in seconds or minutes. A healthy Automatic Call Distribution (ACD) system means calls actually get through and connections remain stable. People actually talk, rather than hang up after three seconds due to audio loss or disconnection.

We recommend to consider a short ACD VoIP interval combined with a decent ASR metric as a clear warning sign. This often indicates that a connection exists, however, it interrupts almost immediately.

Point 3: Post-Dial Delay as an Indicator of VoIP Connection Quality

PDD is the time between you dial and hear ringback or till the recipient answers it. Anything that feels like a pause before a call center agent hears the other end ringing is PDD in action. This metric quietly shapes agent productivity for outbound-heavy operations. A one-second delay repeated across thousands of daily calls adds up to real lost hours.

DSOme specialists describe PDD VoIP as call setup time, since it covers everything that happened before the actual conversation begins. Also, it's useful when you track it over time. It helps flag congestion before it escalates into outright dropped calls.

Point 4: Network Effectiveness Ratio as Less Used but Informative Metric 

NER is one VoIP quality metrics that refines what ASR only hints at. It calculates the ratio of calls that either connected or received a legitimate network response (busy, no answer) against total attempts. Also, it filters out failures caused by the network itself. In short, NER isolates carrier-side reliability from subscriber behavior.

NER VoIP report is one of the more underused VoIP performance metrics in smaller operations. It's because it requires to read SIP response codes rather than just tall connections. 

Point 5: Mean Opinion Score as an Indicator of the Connection

MOS scores are VoIP quality metrics that track voice quality on a 1–5 scale, traditionally gathered from human listeners. However, now mostly estimated algorithmically via tools tied to the R-Factor. A MOS above 4.0 is generally considered toll quality. So, anything under 3.5 starts to sound noticeably rough, with choppy audio or robotic artifacts.

Codec quality plays a direct role here too. G.711 typically delivers noticeably cleaner, higher-MOS audio than compressed codecs like G.729, especially on already-congested links.

Learn Packet Loss, Jitter and Latency as Valuable Factors

These three sit underneath MOS VoIP quality metrics as the raw causes of poor voice quality. 

Our advice: keep an eye on RTP quality. Its packet loss, VoIP jitter and latency together, rather than in isolation. 

What Are the Most Important VoIP Quality Metrics?

Which VoIP metrics should I monitor? Start with this core set: ASR, ACD, PDD, NER and MOS. This combination covers call success, call stability, connection speed, and actual audio quality. 

Reasons Why High ASR Doesn't Always Shows Quality Route

Here's where a lot of buyers get tripped up. A route can post 55% ASR and still deliver garbage audio, dropped mid-call, or one-way audio. ASR only confirms that calls receive answers. So, it says nothing about what happened after that. 

However, you need to understand how to measure VoIP route quality properly. This means you should expand your analysis beyond ASR to a full analysis of VoIP route quality.

For voice-heavy traffic like call center campaigns or customer support lines, MOS and ACD deserve just as much weight as ASR. It’s since a short, choppy call that "counts" as answered still damages the customer relationship. For SMS-gateway or verification-code traffic passing through voice channels, ASR and PDD matter more than MOS. It happens because the goal is fast, reliable connection rather than sustained audio clarity. 

For wholesale resale routes, NER is often the more trustworthy number. It strips out subscriber-side no-answers and shows what the carrier itself is actually responsible for.

The practical takeaway: match the metric to the traffic. A route optimized for one use case can look mediocre by another use case's standards. And that's not a contradiction. It's just a different KPI mattering more.

FAQ: VoIP Quality Metrics

What Causes Low ASR in VoIP?

There are some common causes like incorrect or outdated number formats, congested routes, and carrier blocking. There could also be no answer from the recipient. However, this isn't a technical issue.

What Is ACD in VoIP and How Is It Calculated?

ACD is the VoIP quality metrics that shows the average duration of a successful connection. Divide the total call time by the number of successful calls to calculate it correctly.

What Does a Low Average Call Duration Indicate?

Causes of low ACD can include dropped calls and poor audio quality. These factors lead to negative conclusions and indicate poor call quality with the client, not just technical issues.

What Is PDD in VoIP and What Is an Acceptable Post-Dial Delay?

We've already explained that PDD is the interval between dialing and answering a number. A response time of less than 3 seconds is considered good for most B2B situations. However, anything over 5-6 seconds is considered poor.