Mahalo for the fast response! Im looking forward to trying the music. One line is good for now. I do have WE 400H cards. The great click click is the sound you hear when you are on hold. Almost like a winking but the on hold party can hear it. I wasnt sure if the 551 was capable of it or was it just something for old very large key systems. Looking forward to music soon with your direction. Aloha from Hawaii! David