The "send to Voice mail" Button act as a blind transfer but directly on the user Voicemail this can be useful if the user is busy or unavailable.

To enable the send to Voice Mail feature on the extension button, in /etc/asterisk/iax.conf  or sip.conf the users must have the 'mailbox' parameter defined or be the same as user name/number.

[extension]
host=dynamic
qualify=no
notransfer=no
username=extension
secret=xxxxxxxxxx
type=friend
auth=md5
context=your extensions context
dtmfmode=rfc2833
callerid=name <extension>
mailbox=extension@your voicemail context       ;(es. 300@context)
disallow=all
allow=gsm
deny=0.0.0.0/0.0.0.0
permit=192.168.1.0/255.255.255.0

 and in your extension.conf

exten => _3XXX,1,Voicemail(${EXTEN:1})
exten => _3XXX,2,Hangup()

* The 3 prefix (or what you define) must pe placed in the VoiceMail phone setting in the field "Send to VM prefix"