What is an ideal CST of VoLTE call?

Hi Experts.

What is an ideal CST of VoLTE call?

2-3 seconds.

Thanks @RFSpecialist for getting back.
And it will be form UL INVITE till DL 180 Ringing ?

Invite to 180 ringing difference sometime high value observed due to various SIP reason.

Yes, but alerting phase is 180 ringing?
What SIP to follow then?

Without Prack and Update procedures possible cst ~1sec

CST is Time between the start of the call and the moment the phone of the called party starts ringing.

CST Healthy range is <2.5s

Formulae ( Sec )

: Avg of (Time(180 Ringing) - Time( SIP INVITE Request ) )

Impact: High CST will lead to delays & Silence period before Ring; High CST will obviously will deteriorate user experience.


Image source:

2.1 Seconds for VoLTE-VoLTE calls
this can be achieved by:

  1. Preconditions removal (Update message to be removed).
  2. RX Settings to be final (Activate dedicated bearer at the INVITE message)
  3. Disable DRX at call setup .
  4. VoLTE specific inactivity timer at the Call end (to avoid RRC connection setup in the following Call)

Which vendor(s) allow to configure these 4 settings?