Voix Manager don't require any particular configuration or settings to work, you heve just to follow few conventions:

Extensions

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

example of Extension in you SIP or IAX.conf:

[123]                                 ;<< The extension must be numeric
host=dynamic
username=123
qualify=no
notransfer=no
secret=xxxxxxxxxx
type=friend
auth=md5 ;<< Use md5 and not plaintext
context=your extensions context
dtmfmode=rfc2833
callerid=pippo <123>           ;<< The cli must be in this format
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

sip.conf

In the general context the option "callevents=yes" must be present.

iax.conf

In the general context the option "transfer = no" must be present.

Note: Asterisk 1.2.xx use the equivalent "notransfer = yes".

Free PBX users

Select the option Use FreePBX in the Manager settings .

Codecs

Check that the right codecs are selected for all of your pbx extensions (sip, iax ecc) expecially if you have lost audio during call transfer.