The PreAnswer element answers the incoming call in early media mode. This is useful when you want to play custom caller tunes or to speak specified text dynamically while the call is still in an unanswered state.

You can nest Speak, Play, Wait elements within the PreAnswer element. Refer

To increase the accuracy of the Pre-answer there is nothing much we could do here as it's not guaranteed to work in every destination and country as it's on a best effort basis and it is not supported by few carriers as they do not guarantee delivery of early media to prevent abuse. 

Also, WebRTC SDK does not support early media(PreAnswer) as it's not a part of the WebRTC specifications.