You can make local modifications to this script by copying it from /usr/sbin/vboxmail to /etc/isdn/vboxmail, and editing the copy there.
Note that there is an enhanced version of this script available as
/usr/share/doc/isdnvboxserver/vboxmail.enhanced . To use it, copy it
to /etc/isdn/vboxmail . This enhanced version will send the recorded message
as a MIME attachment in the email notification.
If lame or xingmp3enc are installed in /usr/local/bin, then the message will
be converted to MP3 format; otherwise, it will be sent as a WAV file.
For this to work, you also need the following packages installed:
mime-construct sox
and of course an MP3 encoder if you want MP3 format. Note that the MP3 format
will only be 50% smaller (instead of the 90% you might otherwise expect),
because of the relatively bad sound quality to begin with (8kHz sampling, mono).
If you want to use another MP3 encoder than lame or xingmp3enc, you'll have to
make patches to the script. I'll be happy to implement your changes. Send them
as a wishlist bug report on isdnvboxserver.