Perhaps this thread for background,
but in your case instead of emailing it, then you will need to build a call.file with the correct permissions and then mv it to /var/spool/asterisk/outgoing. You could use the “application” flavor of a call.file perhaps with Playback(${YOURFILENAME})
http://the-asterisk-book.com/1.6/call-file.html (but will still work in later asterii)
A real call.file might look something like
Channel: Local/yourgooglevoicenumber@from-internal
Application: PlayBack
Data: silence/2&/var/spool/asterisk/monitor/2014/09/03/somethingappropriate_20140903-153712-1409607431.13