Voicemail auto delete after 30days

Is there a setting on the GUI where to set 30days voicemail retention?

I did not personally find an option but what I’ve set up is a cronjob to remove automatically at 12:30am nightly.

30 0 * * * /usr/bin/find /var/spool/asterisk/voicemail -type f -name msg???.??? -mtime +7 -delete

you wanting to retain 30 days would do this:

30 0 * * * /usr/bin/find /var/spool/asterisk/voicemail -type f -name msg???.??? -mtime +30 -delete

Give that a try.