Make calls without register

Hi all!
Perhaps a newbi question but I need a solution.
This is the problem. I have a customer that run a software in his callshop. The software is a simple sipproxy and a billing engine for local use. The proxy then send callst to a provider using sip. The proxy is configured with a username which is an extension on my asterisk. How do I configure the Asterisk to allow incoming calls from this customer?
His proxy is behind nat and on a dynamic public ip.

Hope anyone understand


If the proxy doesn’t know how to send a secret, try set

Otherwise simply let the proxy register in your server.