These are optional and are generally unused. There may be cases where you need to set these items at a user level.
serveremail
This setting can be used to identify the source of a voicemail notification message. The value is a string which can be encoded one of two ways. If the string is of the form someone@host.com, then the string will be used as the source address for all voicemail notification emails. If the string is of the form someone, then the host name of the machine running Asterisk will be postpended to the string after insertion of a '@'.
tz
Time zone
imapuser
Imap username if imap storage is used
imappassword
Imap password if imap storage is used
review
Sometimes it is nice to let a caller review their message before committing it to a mailbox. This setting takes a yes or no value. If set to yes, then the caller will be asked to review the message, or save it as is after they have pressed '#'. If set to no, the message will be saved and the voice maill system will disconnect the caller. The default value for review is no. This setting is only available in versions of Asterisk with advanced voicemail feature support.
maxmsg
This limits the number of messages in a voicemail folder. The maximum value is 9999 (hard coded) and the default 100. When a mailbox has more than this number of messages in it, new messages can not be recorded and vm-mailboxfull is played to the caller. No more messages possible is also logged. This setting was introduced in 1.2. In previous versions it was hard coded to 100.
volgain
Set the gain to use when sending a recorded message via email attachment. Requires sox.
operator
This setting enables the user to reach an operator during the time the voicemail message is being recorded, or once a voicemail message has been left, if the review option has been set to yes. This setting takes a yes or no value. The operator must be specified at extension 'o' in extensions.conf, in the context you specify with the voicemail.conf option exitcontext(see below). This setting is only available in versions of Asterisk with advanced voicemail feature support.
callback
Specify a context to be used from the "return phone call" feature in the advanced voicemail features menu. This setting takes a string value set to the outgoing context to be used. The default value for this setting is an empty string. This setting is only available in versions of Asterisk with advanced voicemail feature support.
dialout
Specify a context to be used from the "place an outgoing call" feature in the advanced voicemail features menu. This setting takes a string value set to the outgoing context to be used. The default value for this setting is an empty string. This setting is only available in versions of Asterisk with advanced voicemail feature support.
exitcontext
Optional context to drop the user into after he/she has pressed * or 0 to exit voicemail. If not set, pressing * or 0 will return the caller to the last context they were in before being sent to voicemail (assuming that context has a 'a' or 'o' extension).