Play a sound file to the callee before the actual call start

[tomeko]

@jeroenvip

call start is defined as “when the calle picks up to answer the outoigng call from PBX to callee”

Some phones support passing url to audio file that is used as ring.
Some phones are able to answer with SIP/183, starting early media. I’m not sure about audio here though as I’ve used it mostly for video preview from doorphones before answering.