As reamon points out, FTP reply codes are NOT OS dependant.
If you talk about the text message written after the return code, that is another history.
The messages are neither OS dependant, but FTP server vendoer/version dependant. After the return code each FTP server can write any text they wish.
You just need to take care of the return code, and in the RFC you get them plus what they mean.
#Integration-Server-and-ESB#webMethods#webmethods-Protocol-and-Transport