First, find yourself some Funny Answering Machine messages or other such thing that you want to rotate around. These will usually come in .wav or .mp3 format, which you'll have to convert, using information from voip-info on converting from wav to gsm for use with asterisk.

After you've converted your files, you can follow these simple steps to get some random voicemail.

First, make a directory for your random voicemail gsm files

# mkdir /var/spool/asterisk/random-vm

Now, copy all your gsm files to this directory

# mv /home/admin/*.gsm /var/spool/asterisk/random-vm

Now, delete your old unavailable messages from your voicemail directory. In this case it's extension 7000, yours might differ.

# cd /var/spool/asterisk/voicemail/device/7000/
# rm unavail.gsm unavail.wav unavail.WAV busy.gsm busy.wav busy.WAV

Now, create a symlink from busy.gsm to unavail.gsm

# ln -s unavail.gsm busy.gsm

Now you just need to setup a little script to randomize the voicemail greetings.

# cd /var/spool/asterisk/random-vm
# nano vmscript.sh

And using your favourite editor (nano for us.. yes, we're not that nerdy) paste the following in.

#!/bin/sh

# number of voicemails to rotate is 6 (0-5). Change to match yours.
number=`expr $(date +%S) % 6`

# remove the old voicemail message
rm /var/spool/asterisk/voicemail/device/7000/unavail.gsm

# link the unavailable message to one of the random ones, yo!
ln -s /var/spool/asterisk/random-vm/unavail-$number.gsm /var/spool/asterisk/voicemail/device/7000/unavail.gsm

# done!

Now, exit and save the file. Remember to change the paths, and amount of voicemail greetings as required.

All you should have to do now, is change the permissions, and then add it to cron and you'll be set.

Change permissions like this

# chown -R asterisk:asterisk /var/spool/asterisk/random-vm
# chmod +x /var/spool/asterisk/random-vm/vmscript.sh

Add it to cron like this

# crontab -e

then add the following line

# to rotate voicemail messages
*/1 * * * * /var/spool/asterisk/random-vm/vmscript.sh

Exit and save your crontab. Which will rotate them every minute. Obviously, if you get like 30 calls a minute you may run into trouble, but for a simple house line, this works great, and always keeps callers guessing!

UPDATE

turns out it wasn't fully randomizing the messages as I intended. To get this to work, simply change the script above from

#!/bin/sh

to

#!/bin/bash

and then change

number=`expr $(date +%S) % 6`

to

number=$(($RANDOM % 6))

Now it'll rotate as expected.

UPDATE

This update for Asterisk includes a security fix for chan_iax2. Please see the associated security adivisory for more details:

http://downloads.digium.com/pub/security/AST-2009-001.html

These updates are a fix to a previous security release (released as versions 1.2.31, 1.4.22.1, and 1.6.0.3).

The new versions are being released after additional testing revealed some issues with the way that scanning for users was blocked. Those issues have been corrected in this release.

This security issue affects the 1.2, 1.4, and 1.6 series of Asterisk.

Also note, that Asterisk 1.6.0.4-rc1 was released yesterday prior to the security update. That release has been removed as there will be no 1.6.0.4 release, but rather will be reincarnated as 1.6.0.6-rc1. The reason for the dead release is to avoid 5 digit release numbers.

ChangeLogs for the various releases are available at:

http://downloads.digium.com/pub/asterisk/ChangeLog-1.2.31.1

http://downloads.digium.com/pub/asterisk/ChangeLog-1.4.22.2

http://downloads.digium.com/pub/asterisk/ChangeLog-1.4.23.1

http://downloads.digium.com/pub/asterisk/ChangeLog-1.6.0.5

Thank you for your continued support of Asterisk!

This update for Asterisk includes a security fix for chan_iax2. Please see the associated security adivisory for more details:

http://downloads.digium.com/pub/security/AST-2009-001.html

This security issue affects the 1.2, 1.4, and 1.6 series of Asterisk. Asterisk releases 1.2.31, and 1.4.22.1 only contain the security fix.

Asterisk release 1.6.0.3 has additional changes from 1.6.0.2, including a fix to the Makefile that caused menuselect to break in certain cases.

Also, some issues related to memory leaks and more appropriate SIP dialog responses when receiving 4XX messages have been fixed.

ChangeLogs for the various releases are available at:

http://downloads.digium.com/pub/asterisk/ChangeLog-1.2.31

http://downloads.digium.com/pub/asterisk/ChangeLog-1.4.22.1

http://downloads.digium.com/pub/asterisk/ChangeLog-1.6.0.3

Thank you for your continued support of Asterisk!

This update for Asterisk includes a security fix for chan_iax2. Please see the associated security advisory for more details:
http://downloads.digium.com/pub/security/AST-2008-012.pdf .

This security issue affects only the 1.2 series.

Thank you for your continued support of Asterisk!