The first implementation of External Media is up on Gerrit for review and testing.
More information is on the wiki...
https://wiki.asterisk.org/wiki/display/AST/External+Media+and+ARIThe capability will be in 16.6 but since we're past the feature freeze date for Asterisk 17.0, the capability will be in Asterisk 17.1.
Within the next few weeks, we should also have a sample ARI application available that sends the media off to Google for transcription and just dumps the text to a file.