Post-dial delay (PDD) is the time between sending a call and hearing it ring. Long PDD is usually caused by too many carriers in the chain, failed attempts being rerouted, slow number lookups or network problems at the destination, and it is fixed by measuring PDD per route, shortening the chain and tuning your own failover timers.
How PDD is measured
In SIP, PDD is the time from your INVITE to the first ringing indication from the far end: a 180 Ringing or a 183 Session Progress carrying early media (the ringback tone). Some systems measure to the answer or the final failure instead, so make sure both sides use the same definition. PDD is introduced together with the other core metrics in ASR, ACD, PDD and NER explained.
A few seconds is normal on international routes. What matters most is the comparison: the same destination through two carriers, or the same route today and last week. Callers start hanging up when they hear silence for too long, so high PDD also lowers ASR.
Common causes
| Cause | How it adds delay |
|---|---|
| Long carrier chain | Each hop processes the INVITE and may do its own routing and lookups |
| Rerouting after failures | A carrier tries supplier A, waits for a timeout or error, then tries B and C |
| Number lookups | Portability or HLR queries before routing, see number portability and routing |
| TDM legs and transcoding | Conversion between SIP and legacy networks, see TDM vs VoIP routes |
| Destination network | Paging a mobile that is in poor coverage or switched off takes time |
| Congestion or CPS limits | Calls queue when a carrier is near its capacity, see channels and CPS |
| DNS or SIP timers | Slow DNS resolution or retransmissions over a lossy link |
How to find where the delay comes from
- Split PDD by outcome. Calculate it separately for answered calls and for failed calls. High PDD only on failures often means rerouting inside a carrier.
- Split by breakout. If only one mobile operator is slow, the problem is likely at the destination or in that operator's routing.
- Read a SIP trace. Look at the time to 100 Trying (your direct carrier responding), then the time to 180 or 183. A slow 100 Trying points to your link or the first carrier; a quick 100 and a slow 180 points further down.
- Check your own system. Your switch may itself wait several seconds per route before failing over, adding delay you then blame on the carrier.
- Compare carriers on the same numbers. Send identical test calls through two routes at the same hour.
Fixes
- Ask the carrier how many hops the route has and whether it is direct to the operator. Premium routes usually have fewer hops; see premium vs standard routes.
- Tune your failover: set a reasonable timer for the first ringing indication instead of waiting for long SIP timeouts, and limit how many routes you try per call.
- Make sure 183 with early media is passed through, so callers hear ringback instead of silence.
- Remove unnecessary transcoding and use a codec the whole chain supports.
- Move traffic away from routes whose PDD rises at peak hours, a sign of congestion.
PDD in agreements
PDD is one of the few quality metrics a carrier can control, so it is a fair item in a quality agreement. Agree the definition (to first 180 or 183), the measurement source and the period, and monitor it per route as described in monitoring route quality with CDRs.
If PDD on a destination stays high, ask for a more direct option. VoIPMatcher sources wholesale VoIP termination and manages the route with the supplier: request a VoIP route.
Frequently asked questions
What is the industry standard for post dial delay?
There is no single binding standard. On international wholesale routes, a PDD of a few seconds, roughly up to 3 to 5 seconds, is generally treated as normal, while consistently longer delays point to extra hops, rerouting or slow lookups. Agree the definition and the target with each carrier rather than relying on a general figure.
Does PDD include ringing time?
No. PDD ends at the first ringing indication, a SIP 180 Ringing or a 183 Session Progress with early media. The time spent ringing after that is not PDD. Some systems measure to the answer or to the final failure instead, which produces larger numbers, so check which definition a report or a carrier uses.
Why is post dial delay higher on mobile numbers?
Mobile networks may need to page the handset, which takes longer when it is in poor coverage or switched off, and calls to mobiles often involve portability lookups before routing. PDD on failed mobile calls can be especially long. Split PDD by breakout and by outcome to see whether one operator is the cause.
Does post dial delay affect ASR?
Yes. Callers and dialers hang up when they hear silence for too long, so calls that would have been answered are abandoned before the phone rings. A route whose PDD rises at peak hours often shows falling ASR at the same time, which is a sign of congestion or rerouting along the chain.
Need a route?
Send the destination, route type and target rate. We reply with what we can deliver.