http://www.freepbx.org/support/documentation/howtos/how-to-make-multiple-extensions-use-a-common-voicemail-box

Here, we link the shared mailbox 201 to another extension (202) so that both extensions share the same voicemail mailbox.

Log into the PBX as 'root'.

cd /var/spool/asterisk/voicemail/default

If the voicemail mailbox 202 already exists:

mv 202 202.orig

then

ln -s 201 202