From VTwiki

Recommendations

Here are recommended settings for a Trixbox server:

Setup > Trunks > Add SIP Trunk

Dial Rules :  1YYY+NXXXXXX   (Where YYY is, is your area code)

Trunk Name:  viatalk

PEER Details:

authuser=15554441234
canreinvite=yes
context=from-pstn
dtmf=inband
dtmfmode=inband
fromdomain=chicago-1e.vtnoc.net
fromuser=15554441234
host=chicago-1e.vtnoc.net
insecure=very
nat=no
qualify=yes
secret=YOUR PASSWORD
type=peer
username=15554441234

 

USER Context:  15554441234  (your actual 11 digit phone number)

 

USER Details:

context=from-pstn
secret=YOUR PASSORD
type=user

 

Registration String

15554441234:MYPASSWORD@chicago-1e.vtnoc.net/15554441234

 

You will also want to add an inbound route.