Did you do the test from the same machine as where you are trying to save the service call? Because if you run the test directly on the SD server it might work there but not on a client. In that case it might be related to active/passive FTP.
The explanation is that client does not send attachments to server through the connection it has with server (port 30999 usually) but connects directly to FTP server (port 21 usually).
So the problem here is that the user in Mexico has to be able to log into that FTP server. That means the user needs to be able to resolve the FTP's hostname to IP address and connect to FTP's port. So the issue is probably in the network inbetween.