Post dial delay (PDD): definition and a practical example
Short answer: Post dial delay (PDD) is the interval between the moment the call is requested, when dialing ends, and the first sign that the destination is being called: the ringing or the opening message the caller hears.
In Portuguese: Post dial delay (PDD)
What is post dial delay?
Post dial delay, or PDD, is how long the caller waits in silence after dialing. It starts when the call is requested, as dialing ends, and stops when the first sign arrives that the destination is being called: the ringing tone, or the opening message of an IVR or of the carrier. In SIP, it is the time between the INVITE and the first provisional response, such as 180 Ringing or 183 Session Progress. RFC 6076, which defines performance metrics for SIP, calls this time Session Request Delay.
PDD matters more than it seems. After a few seconds of silence, the caller assumes the call is not working and hangs up. For anyone with an 0800 number or a contact center, each abandonment like that is a customer who tried and could not get through.
What increases PDD
- Routes with several carriers in sequence, each adding a little time.
- A slow number portability lookup before the call is routed.
- An alternate route: when the first destination fails, the equipment tries the next one before the caller hears anything.
- Slow DNS resolution or an overloaded edge device.
A SIP trunk can be registered and complete every call and still have a bad PDD, which is why PDD shows up as a customer complaint before it shows up as an outage.
In practice
Here is what the caller experiences on two calls to the same number (illustrative values):
| Call | Last digit | First ring | PDD | What the caller notices |
|---|---|---|---|---|
| A | 0 s | 2 s | 2 s | The call starts normally |
| B | 0 s | 11 s | 11 s | A long silence; many people hang up |
Both calls complete. Only measuring the time to the first ring shows the difference.
How Monitor uses this
Monitor calls each voice circuit the way a customer does, and a call that takes too long to connect ends up counted as a failure, with the SIP code and the Q.850 cause the route returned. Monitor does not publish PDD as a separate metric. Use the PDD in your call detail records to watch the trend, and the test call to confirm, from the outside, that the number still connects. See SIP trunk and VoIP monitoring and synthetic monitoring.
Frequently asked questions
What is post-dial delay and what is normal?
Post-dial delay (PDD) is the silence between the last dialed digit and the first ring. There is no single value in a standard for every kind of call: it is usually a few seconds and varies with the number of carriers on the path and with the number portability lookup. What counts is the circuit's own baseline: if a number that took a few seconds starts taking much longer, something changed on the route.
What increases post dial delay on a SIP trunk?
Routes with several carriers in sequence, a slow number portability lookup, a failure at one destination that forces a retry on an alternate route, slow DNS resolution and overloaded equipment. In each case the caller hears silence between the last digit and the first ring, and many hang up before it ends.