What is the use case of "Out of Dialog Refer CSS" on SIP Trunk?

[Meddane]

What is the use case of "Out of Dialog Refer CSS" on SIP Trunk?

[b.winter]

Similar question here:
https://community.cisco.com/t5/ip-telephony-and-phones/what-is-the-difference-between-reouting-css-and-out-of-dialog/td-p/4437609

[Meddane]

I already seen the CUCM Help Page but not really clear.

[b.winter]

And which part is not clear? What's an "out of dialog refer"? If yes, probably google helps.
The only scenario where I configure it, is in conjunction with Unity. Then this CSS is used, if Unity transfers the call back to CUCM.
In conjunction with CMS, the "Rerouting CSS" is used.

[Meddane]

I googled and I didnt find clear explanation. I seen example that puts a CSS in the Out of dialog refer css field but without giving the reason.

For Unity Connection Transfer Rules, the Rerouting CSS must be used to allow the transfer but I dont think the Out-Of-Dialog CSS is needed in this case because I configured Unity Transfer Rules without it and it works.

[Jonathan Schulenberg]

This allows the SIP server on the other end of the trunk to issue a REFER for a call that it is not involved in, ie out of dialog. The most common (legitimate) example is CUC MWI. Treat this as sensitive; the CSS probably should be limited to on-net destinations.

[Meddane]

But MWI is sent with the SIP NOTIFY message.

[Roger Kallberg]

It does use Notify, but it is sent out of dialogue.

https://community.cisco.com/t5/collaboration-knowledge-base/understanding-troubleshooting-mwi-on-unity-connection/tac-p/3162952