iax.conf 
Paste the following line under the [general] section: 

register => username:password@sw1.link2voip.com

Paste the following at the bottom of your iax.conf file:

; *** Link2VoIP Inbound ***
[FreeWorldTel]
context=inbound ; change me
type=user
auth=md5
username=xxx
secret=xxx
disallow=all
allow=g726 ; set in codec
allow=gsm ; set in codec

; *** Link2VoIP Outbound ***
[FreeWorldTel-Out]
type=peer
host=sw1.link2voip.com
auth=md5
username=xxx
secret=xxx
disallow=all
qualify=yes
allow=g726 ; set out codec
allow=gsm ; set out codec

extensions.conf 
Example dial string for domestic calls: 

exten => _1NXXNXXXXXX,1,Dial,IAX2/USERNAME@FreeWorldTel-Out/${EXTEN}

Example incoming DID response (should be in your inbound context / assuming that your incoming DID is 18005551212): 
exten => 18005551212,1,Answer
exten => 18005551212,2,Wait,2
exten => 18005551212,3,Playback(somefile)
exten => 18005551212,4,Hangup