Methods and apparatus are provided for preventing a third party from
listening to a conversation between at least two participants on a
telephone. The telephone generates an audio stimulus signal that is
presented through a secondary speaker. The audio stimulus signal may be,
for example, pseudorandom noise or a cancellation signal. According to
one aspect of the invention, the telephone ensures that the audio
stimulus signal does not significantly impair the conversation for the at
least two participants. To prevent the third party from listening to the
local portion of the conversation, the audio stimulus signal is
subtracted from the received signal prior to presenting the received
signal to the user. To prevent the third party from listening to the
remote portion of the conversation, the audio stimulus signal is
subtracted from the received signal.