It depends on which FTP server you're using, but in the case of VSFTPD, there is a config file /etc/vsftpd.conf which has a local_umask parameter. (I think most Linux FTP servers have a /etc/xxxxx.conf file where xxxxx is the name of the server.)
Also note that these masks are increasingly restrictive, so the user probably has to start with a UMASK at least as permissive as the one used by the FTP server. (The user's UMASK can generally be set in the .bash_profile, if not in /etc/profile.)
Also, keep in mind that FTP involves a separate login from the telnet session, so a common problem is that the FTP login (saved in your ZTERM or ATE config) is different than the one you were assuming.