I am trying to configure a Grandstream gateway to work with FS.  I can make
outbound calls without a problem.  However, inbound calls are getting a 403
Forbidden from FS in response to the INVITE from the gateway.

Now, the INVITE's from address is the caller's number (e.g. 1112223333),
which ofcourse, is foreign to the FS.  So the FS sends a 407 Proxy
Authentication Required and the gateway uses username "Anonymous" and the
uri "sip:4000@192.168.72.38" (4000 is the destination for all calls from the
gateway).

Is there an example configuration for this scenario?

Thanks and Best Regards,
Jerry