To enable the send to Voice Mail feature on the extension button you have to make a dialplan like this:

Where are defined your extensions the users must have 'mailbox' parameter in the conf file 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
disallow=all
allow=gsm
deny=0.0.0.0/0.0.0.0
permit=192.168.1.0/255.255.255.0

 in your extension.conf

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

The 3 prefix (or what you need) must pe placed in the VoiceMail phone setting in the field "Send to VM prefix", then press the apply button to save the changes.

To send an active call directly to the user voice mail press the button

on the Extension Button

See also how to configure your Voice Mail ...