Misc. Dialplan Tools pre answer

From FreeSWITCH Wiki

pre_answer is equivalent to a sip status code 183 with sdp.

It establishes media (early media) but does not answer. You can use this for example to send an in-band error message to the caller before disconnecting them (pre_answer, playback, reject with a cause code of xxxx).

That being said, most apps that require media (like playback) should do a pre_answer for you. If you are having an issue with a piece of equipment taking a long time to setup a media path, you could do pre_answer, then sleep in the dial plan.