PURPOSE: To prevent crosstalk during calling to use an omnibus line by prohibiting self-calling operation with a calling operation prohibiting means from the time of a calling start by any other telephone set detected by a calling start detecting means to the time of a calling end detected by a calling end detecting means.

CONSTITUTION: A calling start detecting means 1 detects that the calling is started by any one of telephone sets 51∼53 connected to an omnibus line 54 and a calling end detecting means 2 detects the end of the calling in the omnibus line 54. A calling operation prohibiting means 3 prohibits the self calling operation by the detection of the calling start and cancels the prohibition by the detection of the calling end. Thus, during the calling to use the omnibus line, the crosstalk is prevented from being generated by the erroneous calling from the other telephone set.