T48G Intercom Auto-answer

[cm21]

Yealink Support, to answer your questions:
1) I want an intercom button, that also monitors the status of that extension. Before I call the extension, I want to know whether they are already on the phone with someone else. That is why I want it to show the BLF status.
2) I understand the difference in the features. I was asking about how the phone handles the two different types of DSS keys "Intercom" and "BLF". It appears to me that the phone handles them both the exact same way, but just shows a different icon.
3) I understand how an Intercom call should work, but was asking what SIP headers (or similar) the phone needs to see from the server to handle a call as an Intercom call instead of a regular call.

Based on the response from "jolouis" below, he's stating that the auto-answer header (Alert-Info:answer-after=0) is what is needed.
However, I'm wondering if the phones consider "Auto-Answer" and "Intercom" to be two different things, because:
* If I set "Allow Intercom" to Disabled on a phone (and restarted just to be sure), it STILL auto-answers when given that SIP header.
* Also, at least so far, the (new?) features.intercom.headset_prior.enable setting doesn't seem to be having an effect - goal is to force speakerphone for incoming intercom calls, regardless of whether the headset button is currently on/off before the intercom call comes in.