Post-dial delay (PDD) is the measurement of how long it takes for a calling party to hear a ringback tone after initiating a call. Technically it is the delta between the SIP INVITE and a 180 Ringing or 183 Session Progress response.
Callers who experience post-dial delay will report long waits before hearing ringing. In extreme cases high post-dial delay will be reported as dead air, one-way audio, or dropped calls since the calling party has no indication that the call has been properly established, or whether or not the destination has answered.
Tolerance for post-dial delay varies widely by geography. In North America most users will notice post-dial delay of ~5 seconds, but in some locations like South Africa post-dial delay of 10 seconds or more is common and even 15 seconds of silence before hearing ringing can go without remark by callers.
Most post-dial delay is due to improper SIP signaling respones from destination networks due to different interpretations of the SIP RFC. For example, in general a 180 Ringing response from a destination indicates that the calling party network should play the caller whatever their local ringback tone is and a 183 Session Progress from a destination includes specific audio in the SDP to be played back to the caller; however, in some cases a destination may try to provide specific ringback with a 180 Ringing response, or neglect to provide one with a 183 Session Progress response. Further, in some cases the ringback provided may be silence.
Post-dial delay can also be experienced when calling geographically remote destinations from the caller. Non-geographic destinations like satellite phones are also prone to greater post-dial delay.
Twilio's Super Network actively monitors our carrier partners for increases in post-dial delay and will proactively notify and resolve problems that are detected without action required from customers.
In some cases calls to a specific destination may encounter post-dial delay that is not significant enough in aggregate to alert Super Network to the issue; in these instances customers can reach out to Twilio Support for assistance in investigating the issue and escalating to carrier partners if appropriate.