The PreAnswer element answers an incoming call in early media mode — that is, at a time when the calling and answering SIP user agents communicate before the SIP call is actually established. This is useful when you want to play custom tunes or use specified text dynamically while the call is still in an unanswered state.
You can nest Speak, Play, and Wait elements within the PreAnswer element.
PreAnswer is not guaranteed to work for every destination. It’s not supported by every carrier due to regulations and abuse-prevention protocols. Therefore, there aren’t many steps you can take to increase the accuracy of the PreAnswer. Please note that the WebRTC SDK does not support PreAnswer because early media is not a part of the WebRTC specifications.