AT&T SYSTEM 25 Manual page 124

Call Waiting
Neither SLACs nor DTACs can have the Call Waiting feature assigned to them. However,
other voice terminals in the system may have this feature assigned. If the attendant hears
special ringback, it indicates the voice terminal is busy but the called party has been notified
of a waiting call. Since this station is busy, the attendant may "camp-on" the call to that
station. For more information on this procedure, see the entries for "Camp-On" on page 3-9
and "Return Coverage on Busy" on page 3-45.
Call Waiting 3 - 7