Hi,

If the person you are calling is on the phone and has the Call Waiting facility enabled then you will get the normal ringing tone until they answer. If they do not answer, then the call will be switched through to their Voice mail facility, if they have it setup.

If the person you call is on the phone when you call they will receive a tone to indicate to them that there is another call waiting to be answered. They can choose to either put the current call on hold and answer you or ignore it and you will go through to their Voicemail.

If the person you call doesn't have the Call Waiting facility then you will receive the busy signal if they are already on the phone.