If you have a seperate wav or gsm file you can play it before you send the person to voicemail using the Background(filename) command, after playing the message send the person to voicemail using the Voicemail command with the 's' flag which will skip the standard message. Something like this:
exten => 1000,1,Answer
exten => 1000,2,Dial(SIP/1000,15)
exten => 1000,3,Background(your-message)
exten => 1000,4,Voicemail(s1000)
exten => 1000,103,Background(your-message)
exten => 1000,104,Voicemail(s1000)
In this example the phone would ring for 15 seconds and if there is no answer play your message then send the person to voicemail. Also if the dial command returns a busy it will go to priority N + 101 (which would be 103) and do the same thing.