According to one embodiment of the invention, there is provided a telephone system comprises a plurality of telephone terminals and a main unit. The main unit comprises an authentication processing unit performs login authentication MAC address authentication, and a mode specification unit receives specification of a plural terminal registration mode. The MAC address authentication refuses logins from telephone terminals differing in MAC address from a telephone terminal that has been allowed to log in firstly even if the logins are made by the same extension numbers. The plural terminal registration mode exclusively allows the simultaneous login by the same extension numbers from a plurality of telephone terminals having different MAC addresses. The authentication processing unit gives priority over the plural terminal registration mode higher than the MAC address authentication and makes the MAC address authentication void in the plural terminal registration mode.