Web Service Connection Problem.

Hi,
Was wondering if anybody could help.

I have a webservice that I have developed in VS.NET 2003 using C#.
I've written a Windows form using C# in VS.NET 2005 to communicate with the
webservice.
This appears to be working using the following cases:
Direct Internet Connection
Proxy
Proxy with Authentication.

But a colleague of mind has just tested whilst travelling, and it does NOT
work
when confronted with a wifi credit card payment system.
Has anybody come across this and if so did you find a solution?

Many thanks

Wayne