If you hear a busy tone, and the person you called has enabled call-waiting,
you can press 1 to alert them that someone is attempting to contact them, or
you can whisper in their ear provided you both have the necessary equipment
and phone system software.
If you hear a busy tone, you can press 6. you will hear a confirmation
tone, hang up. When they hang up from their call, your phone will ring
and if you pick up your handset, your phone will automatically call
them. See Automatic Callback Busy.
For standard single-line or cordless phones.