http://www.voip-info.org/wiki/view/Asterisk+config+voicemail.conf

These are all settings in /etc/asterisk/voicemail.conf.

Adjust for your needs. Start with 2 or 3 and go up:

minsecs=4

Normalize voicemail attachment volume:

volgain=6.0

emailbody=Dear ${VM_NAME}: Just wanted to let you know you were just left a ${VM_DUR} long message (number ${VM_MSGNUM}) in mailbox ${VM_MAILBOX} from ${VM_CALLERID}, on ${VM_DATE}, so you might want to check it when you get a chance. Thanks!\t\t\t\t--the phone system

emailbody=New voice message:\n\nTo:          ${VM_NAME}\nDuration:    ${VM_DUR}\nMsg Number:  ${VM_MSGNUM}\nMailbox:     ${VM_MAILBOX}\nFrom:        ${VM_CALLERID}\nDate/Time:  ${VM_DATE}