>Another possible cause could be that the customer network admin says that "SQL Service is not listening on tcp 1433". How do you set SQL Server to listen to this port?

It's in the configuration tool, server protocols, rightclick on TCP/IP, check properties - probably under some advanced button or 2nd page. There's also one of the values just above the port, probably line 3 in each port's section, which is blank but needs to be zero if you want this to work (which I googled out when I needed it, and of course can't find again)

Even better, if the admins frown at the usage of 1433, which every kid knows is the SQL port, try to agree to some other port instead, and activate that port. Then in your connectstring the server section becomes "...;Server=ServerNameHere,12345;" assuming 12345 is the port you opened.