Calling system on IAX through Asterisk to phonesystem on SIP

We are trying to get our calling system to connect to our helpdesk through Asterisk.
We are using FreePBX

Helpdesk can utlilize 8 channels and the system is set up like this.
Callingstations => callingcentral=>Asterisk=>AlcatelSIPcentral=>Helpdeskphones.
And vice versa.

How do I set up Asterisk to handle the IAX calls from Callincentral through to the SIP sentral and Helpdesk phones.

We are located in Norway, in case that is a crutial info.

All input is much appriciated.

Thanx all.