Trunks DID etc

I need to setup a callshop on my asterisk. What i want to do is a Trunk -that i already did - to use at the callshop program to regster on asterisk. What i want is when a phonecall is made from the Callshop (SIP) to come to asterisk and from there to use on of the trunks i already setup for a provider.

Now when a call is made the phone comes to asterisk but a local extension is ringing.

Must Be Like This

SIP PHONE > CALL SHOP SIP & BILLING PROGRAM > ASTERISK > PROVIDER

Can you give me a step by step info on how to do this ?

Can you post what you get in the CLI when the call comes in ? It could be the billing software is messing with the number. Also is the billing done on the asterisk box and what are you using for billing ?