TelFree Asterisk SIP Trunking Example: 

sample config from /etc/asterisk/users.conf (Asterisk 1.4):

[trunk_1]
disallow = all
allow=g729
context = DID_trunk_1
dialformat = ${EXTEN:1}
hasexten = no
hasiax = no
hassip = yes
host = sip99.telfreesa.com
port = 5060
registeriax = no
registersip = yes
secret = xxxxxx
trunkname = Custom - TelFree
trunkstyle = customvoip
username = 8775xxxxx
callerid = 8775xxxxx
fromuser = 8775xxxxx
fromdomain = sip99.telfreesa.com
permit = 196.46.138.29/29
insecure = very
qualify = no
canreinvite = no

You can do most of this using the GUI (http://www.asteriskguru.com/tutorials/asterisk_gui.html) and then add a few things after the basic setup.

You will need to purchase a G.729 license (+/- $10) for the best voice quality and codec results, from: (http://www.digium.com)

NOTE: Please be certain to replace the "xxxx"'s above with your personal details.