A PBX connected to a public exchange includes a separate caller ID service unit for providing a caller ID and/or a system tone by performing a digital signal process. To display the caller ID on a receiver terminal, or caller ID phone, a line is established with the PBX through general switched telephone...http://www.google.com/patents/US20040208300?utm_source=gb-gplus-sharePatent US20040208300 - Apparatus and method for transmitting-receiving CID in a PBX
