FileMaker and Firewalls

I have a Networking question that may be obvious to Security Admins, but not so obvious to the rest of us.

I have a distributed FileMaker solution that communicates back to my server over a Wide-Area-Network. The customer network has a hosted solution running on FM 10 Server with a secure firewall which has an HTTP Proxy filter turned on. My Sever resides on my business network which is also fire-walled, but I am not using an HTTP Proxy server. My server IP has a Public IP. I have provided my Customer with my Server IP and the IP of the Public URL to enter into their HTTP Proxy filter list.

Here is the problem. When the customer network initiates a script from their FileMaker solution they cannot communicate to a Public Web Address called in a script from my Sever.

So the process is:

1. Customer hosted solution calls script to my remote server outside their firewall

2. My server activates the called script. The remote FileMaker solution has a data source that points to the public domain of my server

3. My server initiates a script to a public URL to retrieve text

4. The retrieved text is then returned to the original hosted file via a Get(ScriptResult) return

This process stops at step 3 because of the Customer's HTTP Proxy filter.

Would you expect this kind of result from the Network? Have you seen this before?

This is an interesting problem that goes beyond normal FileMaker Networking. Any tips?

Thanks.