According to the present invention, a voice storage device includes a port
control unit for monitoring a service state of a port of a PBX extension
circuit, and a private branch exchange is configured to transmit, upon
receipt of an incoming call, an incoming signal including caller
information and the number of a port receiving the incoming call to the
voice storage device without bringing an outside line to an off-hook
state. As the voice storage device receives the incoming signal, the port
control unit detects the port, which is available. When the port is
available, paging directed to an individual extension telephone is
performed through the private branch exchange by use of this available
port, and until the outside line goes off-hook, a caller keeps hearing a
ringback tone with the ringing signal being sent from a telephone office
to the caller.
