PDDAlso known as Post-Dial Delay, Post-Dialling Delay

How PDD is measured

On a SIP network, PDD runs from the initial INVITE to the first 180 or 183 response; some tools also report time to 200 OK separately. It is usually quoted as an average per route or destination, in seconds.

Every hop adds to it: each transit carrier’s softswitch routing decision, each failover attempt after a rejection or timeout, database lookups such as number portability checks, and the destination network’s own call set-up, including paging a mobile phone.

Common causes of high PDD

• Long route chains with several carriers in transit.
• Failover retries after timeouts on the first choice route.
• DNS or authentication delays on the originating side.
• Mobile network set-up time, which is naturally longer than for fixed lines.
• Gateways waiting for a free line or re-registering.

Keep failover timers tight, prefer routes with fewer hops, and watch PDD alongside ASR and ACD: rising PDD often precedes falling ASR, because callers abandon.

PDD and mobile gateways

Where a network returns 183 Session Progress with early media, the caller hears tones or announcements immediately, so measuring PDD to that first 183 reflects what callers actually experience.

Calls out through a GSM gateway include mobile call set-up time on the radio network. Sending early media (183 with ringback) promptly reassures callers. GSMCalls records call timings in CDRs so PDD can be compared across sites and devices; see features.