A technique for notifying a person who is engaged in a telephone call with a second person that a third person is calling him or her is disclosed. The notification comprises not a series of beeps but a voicesynthetically generated or pre-recordedthat has at least one different psychoacoustic characteristic...http://www.google.ca/patents/US6687338?utm_source=gb-gplus-sharePatent US6687338 - Call waiting notification
