A communication system shares terminal identification information (TII) used for identifying a terminal that sends a spam phone call, with another communication system belonging to another network provider, and includes: a storage section that stores the TII of a terminal that sends a spam phone call; a transmission section that transmits the TII stored in the storage section to the other communication system; a reception section that receives the TII used for identifying the terminal that sends the spam phone call; a registration section that registers the TII received by the reception section in the storage section; and a spam phone call determination section that determines whether there is a match between terminal information indicating information of an outgoing call terminal that sends a connection request, and the TII stored in the storage section, thereby determining whether the outgoing call terminal sends a spam phone call.