There are just few conventions:

1) the extension must be numeric es. [123]
2) the callerid must be in the format "name <extension>"

example in you SIP or IAX.conf:

[123]
host=dynamic
qualify=yes
notransfer=no
secret=xxxxxxxxxx
type=friend
auth=md5
context=your extensions context
dtmfmode=rfc2833
username=123
callerid=pippo <123>
mailbox=123@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

The extensions in Voix Manager are represented by buttons.

The Extension Button have 11 States. The off hook state is only available for zap channels. Pointing the mouse over the CallerIdName or the Voice mail icon will be displayed the relative ToolTip Right clicking the button will show the context menu with all the commands available Clicking the CallerIdName will be displayed the user detail window