In order to make your Asterisk pbx "talk" to iTalk, you need to define an instance for it in your sip.conf so you can reference it to dial outside numbers eg.
[iTalk] type=friend secret=$your_italk_password username=649974xxxx fromuser=649974xxxx host=akl.italk.co.nz dtmfmode=rfc2833 insecure=very ;important nat=yes canreinvite=no disallow=all allow=ulaw allow=gsm
Also inside the sip.conf you need to register to the iTalk proxy for ingress calls. eg.
register => $username:$password@akl.italk.co.nz/$extension_to_forward_incoming_calls_to
inside extensions.conf (aside from having a valid extension to forward incoming calls to as specified in the register line above) you need to have rule to push egress calls out the "iTalk" sip define.
I route all egress calls out via it when people 'dial 1 to get out' so my rule inside extensions.conf looks like
exten => _1X.,1,Dial(SIP/iTalk/${EXTEN:1},30,Tr)
If you had other dial out methods for different toll areas you would obviously match it with the "09" prefix, ie.
exten => _09X.,1,Dial(SIP/iTalk/${EXTEN:1},30,Tr)
And when 
Slingshot/iTalk allow registering other area codes you could do your own toll bypass network all for only $10/areacode/month for unlimited calls ;)
ie.
have a declaration for each area code in sip.conf, then route via them based on what the user dials eg.
exten => _09X.,1,Dial(SIP/iTalkAKL/${EXTEN:1},30,Tr)
exten => _04X.,1,Dial(SIP/iTalkWLG/${EXTEN:1},30,Tr)
exten => _07X.,1,Dial(SIP/iTalkHLZ/${EXTEN:1},30,Tr)
exten => _03X.,1,Dial(SIP/iTalkCHC/${EXTEN:1},30,Tr)
2 pages link to AsteriskItalkSetup:
