Post Dial Delay (PDD) is the amount of time between when a caller sends a call request and when the destination side begins ringing or the call receives a response.

In simple terms:

PDD measures how long a caller waits after dialing before hearing ringing, an answer, or another call-progress response.

PDD is an important performance metric for wholesale VoIP carriers, SIP providers, call centers, CCaaS platforms, and telecom operators because excessive delay can create a poor calling experience and may indicate routing or network problems.

How Does PDD Work?

When a customer places a call, several signaling and routing steps may occur before the destination starts ringing.

A simplified process looks like:

Caller Dials Number
↓
SIP INVITE Sent
↓
Carrier / Route Processing
↓
Destination Carrier Processing
↓
Call Progress Response / Ringing
↓
Destination Starts Ringing

The time between the initial call request and the relevant call-progress response is commonly referred to as PDD.

For example:

If a caller sends a call request at 10:00:00.000 and receives ringing at 10:00:01.500, the PDD is approximately:

1.5 seconds

What Is a Good PDD?

There is no single universal PDD value that is considered “good” for every destination.

In wholesale VoIP, carriers generally prefer lower and more consistent PDD, provided the route maintains good ASR, ACD, voice quality, and reliability.

For example:

A route with consistently high PDD may indicate inefficient signaling, routing problems, congestion, or issues with an upstream carrier.

Why Is PDD Important in Wholesale VoIP?

PDD directly affects the caller’s experience.

High PDD can result in:

• Long silence after dialing
• Delayed ringing
• Call abandonment
• Lower customer satisfaction
• Lower ASR in some traffic environments
• Poor call-center agent experience
• Complaints about route quality
• Difficulty identifying routing problems

For high-volume call centers, even a small increase in call setup delay can become noticeable across thousands of calls.

PDD in Wholesale Voice Routing

PDD is particularly important when comparing multiple termination routes.

Suppose a carrier has three routes to the same destination:

Route A: $0.008/min — PDD 1.1 sec
Route B: $0.006/min — PDD 2.8 sec
Route C: $0.004/min — PDD 6.0 sec

A pure Least Cost Routing (LCR) system may prefer Route C because it is the cheapest.

However, a carrier focused on overall route performance may choose Route A because its lower PDD and potentially better call performance can provide a better customer experience.

This is why carriers should evaluate:

Cost + PDD + ASR + ACD + Capacity + CLI + Reliability

rather than looking at termination price alone.

What Causes High PDD?

High PDD can be caused by several factors, including:

1. Inefficient Routing

A call may pass through multiple intermediate carriers before reaching the destination.

More network hops can potentially increase signaling delay.

2. Network Congestion

Heavy traffic or overloaded infrastructure can delay call processing.

3. Slow Upstream Carrier Response

The originating or terminating carrier may take longer to process the call request.

4. SIP Signaling Problems

Incorrect or inefficient SIP signaling can contribute to delayed call setup.

5. Routing Loops

Incorrect routing configuration can cause calls to travel through unnecessary network paths.

6. Destination-Specific Issues

Some destinations or operators may naturally have different call setup characteristics.

7. Media and Signaling Infrastructure

Problems with gateways, SBCs, softswitches, or interconnection points can also affect call setup performance.

PDD vs ASR

PDD and ASR measure different aspects of call performance.

PDD: How long it takes for the call to reach a call-progress response.

ASR: The percentage of call attempts that are successfully answered.

For example:

Route A

• PDD: 1.2 seconds
• ASR: 90%

Route B

• PDD: 5 seconds
• ASR: 55%

Route A would generally appear healthier from both a call setup and answer-rate perspective.

However, low PDD does not automatically mean a route is good. A route should be evaluated using multiple performance metrics.

PDD vs ACD

PDD measures call setup delay.

ACD (Average Call Duration) measures the average duration of answered calls.

For example:

• PDD = 1.5 seconds
• ACD = 4 minutes

This means calls are being connected relatively quickly and answered calls have an average duration of four minutes.

Both metrics provide different information about route performance.

PDD and Call Centers

PDD is particularly important for call centers and predictive dialers.

When a large number of calls are generated simultaneously, excessive setup delay can cause:

• Agents waiting for calls to connect
• Delayed call progression
• Increased abandoned calls
• Inefficient agent utilization
• Poor campaign performance

For this reason, call-center operators often monitor PDD together with ASR, ACD, CPS, concurrent calls, and answer supervision.

PDD and SIP

In SIP-based VoIP networks, PDD is closely associated with the time required for the SIP signaling process to progress.

A typical call may involve:

SIP INVITE
↓
100 Trying
↓
183 Session Progress / 180 Ringing
↓
200 OK

The exact signaling sequence can vary depending on the network and configuration.

When measuring PDD, carriers should clearly define which signaling event marks the end of the PDD measurement, because different platforms may calculate the metric differently.

Does High PDD Always Mean Poor Voice Quality?

No.

PDD measures call setup delay, not the quality of the audio after the call is connected.

A route can have:

• Low PDD but poor audio quality
• High PDD but good audio quality after connection
• Low PDD and high ASR
• High PDD and low ASR

Therefore, PDD should be analyzed alongside ASR, ACD, packet loss, jitter, latency, MOS, and other applicable metrics.

How Carriers Monitor PDD

Wholesale carriers can monitor PDD by analyzing call records and SIP signaling.

Typical monitoring may include:

• Average PDD
• Maximum PDD
• PDD by destination
• PDD by supplier
• PDD by route
• PDD by customer
• PDD by time period
• PDD trends during traffic peaks

For example, a carrier may discover that a particular supplier normally has 1–2 seconds PDD, but increases to 5–6 seconds during peak traffic.

That information can help the carrier investigate congestion or adjust routing.

PDD and Route Optimization

PDD can be incorporated into quality-based routing and best-cost routing.

For example:

Route A: $0.010/min — PDD 1.0 sec — ASR 90%
Route B: $0.008/min — PDD 2.0 sec — ASR 84%
Route C: $0.006/min — PDD 5.0 sec — ASR 60%

A sophisticated routing system may not automatically choose Route C simply because it is cheapest.

Instead, the carrier can establish routing rules that consider price, PDD, ASR, ACD, capacity, and reliability.

Common PDD Questions

What does PDD stand for in VoIP?

PDD stands for Post Dial Delay. It measures the delay between initiating a call and receiving the defined call-progress response.

What causes high PDD?

High PDD can result from inefficient routing, network congestion, multiple carrier hops, slow signaling responses, routing problems, or infrastructure issues.

Is lower PDD always better?

Generally, lower and consistent PDD is desirable, but PDD should not be evaluated by itself. ASR, ACD, voice quality, capacity, reliability, and other metrics also matter.

What is the difference between PDD and latency?

PDD measures call setup delay, while network latency generally refers to the time required for packets to travel between network endpoints.

They are related but are not the same measurement.

Does PDD affect ASR?

High PDD can contribute to poor calling experiences and may be associated with lower ASR in some traffic environments, but PDD itself does not directly determine whether a call is answered.

How is PDD measured?

PDD is measured by calculating the time between a defined starting event—typically the initial call request—and a defined call-progress event, such as ringing or another signaling response.

Best Practices for Managing PDD

Wholesale carriers should:

• Monitor PDD by destination and route
• Compare PDD between suppliers
• Investigate sudden PDD increases
• Monitor PDD during peak traffic
• Combine PDD analysis with ASR and ACD
• Maintain route redundancy
• Avoid relying solely on the cheapest route
• Review SIP signaling when PDD becomes abnormal
• Monitor supplier performance continuously

A strong wholesale voice operation should aim for low, stable, and predictable PDD while maintaining acceptable ASR, ACD, voice quality, capacity, and reliability.