We fought this constantly. There is not a good fix for your problem. If your phones are set to ring first in 10-1 and you are on a CO line because the acutal DN or Intercom key is not being used the intercom call will try to ring the phone with the muted busy ring becuase it is not busy. If you switch the phone to voice first in 10-1 it will always ring busy no matter if you are on a CO line or not. If you can talk the customer into voice first problem solved. If not you can take all of the CO keys off and make them call out on the DN or intercom key by dialing 9 you will get the busy signal you are looking because the DN is busy. However most people like the CO's as buttons on the phone. Same would hold true with pool line keys because the DN is not busy.