For trixbox 2.8:
Click on Trunks, then "Add SIP Trunk".
Under the [Outbound Caller ID] field you need to decide what Callerid you are going to use. If each of your phones is going to use a different callerid - you may leave this blank.
You can use any trunk name, I called it Callcentric.
Under Peer details use:
type=peer
secret=your_account_password
insecure=port,invite
host=callcentric.com
fromuser=1777xxxxxxx
fromdomain=callcentric.com
defaultuser=1777xxxxxxx
context=from-pstn
videosupport=no
canreinvite=no
Use your account password which you use to log in to your account and 1777xxxxxxx is your main Callcentric number associated with your user id.
For User Details - remove all default entries and leave it blank.
For Register String use:
1777xxxxxxx:your_account_password@callcentric.com/1777xxxxxxx
Click Submit, then Apply new settings.
For trixbox 2.6 you can this:
context=from-pstn
fromdomain=callcentric.com
fromuser=1777xxxxxxx
host=callcentric.com
insecure=port,invite
secret=your_account_password
type=peer
username=1777xxxxxxx