When a caller calls a call busy called station, the caller may request that a message be provided to the called station while the caller waits. In such instance, the called station receives the message while busy with the other call. The message may be speech indications provided by the caller which...http://www.google.com.hk/patents/US6735295?utm_source=gb-gplus-share專利 US6735295 - Call waiting messaging
