I tried as you said to connect with my ftp-client at www.testdomain.com
and it worked well. A little bit strange as it sound, because almost all my life i reached ftp-servers with ftp.testdomain.com, but this will works also. Thank's.
But the other thing I don't onderstand well. As I read it well, you say that even if I make an A record (ftp xxx.xxx.xxx.xxx) it will not work on my ISP hostserver because my dns server also is installed on it.
Please correct me if I read above wrong.