Post Dial Delay: why the far end rings late

You dial. The phone stays silent. Your customer hangs up before the first ring. On the report that silence shows up as a lower answer rate, and the cheap route that caused it still looks fine on price.

Post Dial Delay (PDD) is that wait. This article is for VoIP operators who see ASR fall on a destination and need a short checklist: what PDD is, what is too long, what causes it, how to prove it, and when a fake ring is lying to you.

What PDD measures

PDD is the time from the last dialed digit (or the Call / send action) until the caller hears ringback, busy, or another clear outcome.

In SIP terms, count from the outbound INVITE to the first useful provisional or final response: 180 Ringing, 183 Session Progress, or a negative code such as 480, 4xx, 5xx, or 6xx. The same idea appears in RFC 6076 §4.3 as Session Request Delay (SRD).

It can be represented like this:

For SIP, that is the time from sending the INVITE to receiving the first ringing (or negative) response. Here PDD is marked in the call flow with the proxy:

If the caller never hears feedback, they abandon. Those abandoned attempts count against answer-seizure ratio (ASR). Low ASR means more attempts for the same completed minutes, more hunt time on the next terminator, and a worse customer conversation even when your buy rate looked good.

What counts as too long

Tolerance depends on market and destination.

• Many USA and Europe callers get impatient after about five seconds of silence.
• Some regions tolerate 10–15 seconds before ringback.
• Destination profiles differ: one corridor may sit at 3–4 seconds, another at 8–9.

Working rule: under about three seconds is strong; under seven seconds is what many vendors treat as acceptable before they open a ticket; above seven seconds is where you escalate. Our ASR article uses a tighter consumer bar (about 4–5 seconds, under two as excellent). Use both: the vendor SLA number for tickets, the tighter number for customer experience on retail and dialer traffic.

Put the threshold in the interconnect agreement when you can. Then you argue from a number, not from “it feels slow.”

Rule out the phone before you blame the route

Older desk phones wait for a digit timeout (often a few seconds) before they send the number. The caller never presses Call or #. That wait is not PDD. It happens before your switch sees an INVITE.

Confirm the caller pressed send, or shorten the interdigit timer on the device, before you open a vendor ticket.

Likely causes

Too many hops in the chain

Each vendor in the path can spend time deciding whether it can complete the call. Five or six hops add seconds. Prefer a more direct terminator for destinations where silence after dial is the complaint.

Slow or wrong SIP feedback

Some networks send 180 or 183 late, omit early media on 183, or play silence as ringback. The call path may be fine later; the caller already hung up.

Processing and transport delay

Parsing, auth, routing lookups, queueing, and timers on SIP nodes add milliseconds each. Across many nodes and a busy network, those add up. Geographic distance and satellite destinations add more. DNS lookups on hostnames (instead of IPs) can add tens of milliseconds, and in bad cases much more. If you suspect DNS on your side, try the peer as an IP and compare.

Network and wireless issues

Dropped or delayed SIP responses, overloaded vendor gear, or a handset with a weak radio link can leave the caller in silence while the far end has not started ringing yet.

Call forwarding chains

Forward to a toll-free or intermediate number that forwards again and you pay for each hop in setup time.

How to confirm and fix

1. Measure on one destination and one terminator. Compare PDD (or silence-to-ring time from a test phone) against yesterday and against a second terminator. If only one path is slow, you found the candidate.
2. Swap the terminator first. Send a small test volume to another vendor for the same prefix. If PDD drops, the first path was the problem. If PDD stays high, look closer to the destination or further down the chain.
3. Capture a PCAP before you escalate. Send the vendor the trace that shows your INVITE and the late (or missing) 180/183. A PCAP ends the “we see nothing” argument the same way it does when a provider claims they never got your traffic. See when the provider does not see your calls in their CDR. On M4, use the built-in PCAP tools when you need evidence on a live path. The example below shows a PDD on the vendor side:

1. Do not fix PDD with a fake ring. Some cheap or NCLI paths play local ringback while the real far end is still quiet. Your PDD metric looks better. Callers still wait through empty ring and hang up, so ASR stays weak. If you hear early ring with no progress, raise it with the vendor or move the traffic.

Prevention

• Prefer fewer hops on destinations where silence after dial shows up in tickets.
• Track PDD (or ASR drops that smell like abandon-before-ring) per terminator, not only per destination.
• Keep quality next to price when you rank routes. High PDD shows up as bad ASR; quality-weighted routing ranks terminators on live ASR and ACD so a cheap slow path does not stay first forever.
• Put a PDD threshold in the agreement so tickets have a number.

FAQ

Is digit timeout the same as PDD?

No. Digit timeout is the phone waiting before it sends the number. PDD starts after the call attempt leaves the device (after the INVITE in SIP).

Why does ASR fall when PDD rises?

Callers abandon during silence. Abandoned attempts count as unsuccessful for ASR, so the route looks worse even if answered calls sound fine.

What should I send the vendor?

A PCAP of a slow call, the destination, timestamps, and your agreed PDD threshold. Screenshots of “it felt long” rarely move a wholesale NOC.

Does a fake ring fix PDD?

It masks the metric. Callers still wait. ASR and complaints stay. Treat fake ring as a vendor quality issue, not a fix.

Next step

If silence after dial is killing ASR on a wholesale path, measure PDD per terminator, swap the path, and keep a PCAP for the ticket. Then put ASR (and ACD) into how you rank those terminators so the slow cheap route does not win on price alone. Start from The cheapest route that kills your ASR.