Login problem - firewall related?

We have been able to build a proof of concept application, and this seems to be working well on all the tests in our environment.

However, when the customer tries to run the application, it just hangs at the login stage. The customer can access their demo account through an IE browser, and I believe the settings they are providing to web services are the same (and are valid).

I am wondering if there is something in their proxy or firewall which could be blocking the web services communication, but allowing the browser communication? Could this be an explanation, i.e. does web services use a different protocol in some way to the browser (sorry if this is a basic question), or can you assume that if the browser access works, web services should work?