PURPOSE: To deal with a blocking request with no omission of the voices and the data caused in a busy mode by evading the blocking of a single speech channel and instead by treating the speech channels as a group so as to accord with the operation of a system.

CONSTITUTION: When a blocking/canceling request is received, it is decided based on a group number conversion table 1 whether the designated speech channel belongs to a group or not. If so, a blocking request or a canceling request is decided. When a blocking request is confirmed, it is decided based on a group control table 3 whether the group is presently used or not. If so, a face whether an idle group is available or not is checked based on the table 3. If so, the speech channel under use is switched to a speech channel belonging to the idle group. Then all speech channels of the group to which the designated speech channel belongs are blocked. The numbers of all speech channels included in the group are obtained from a speech channel number conversion table 4.