To provide a calling system, a terminal adapter, a calling device, a server, and a calling system which perform automatic switching between calling by line switching and VoIP calling according to the connection state of a calling device at a calling destination in the dial-up connection.

The server 12 registers VoIP calling availability information in a database through steps T2 to T7 when the received signal from the terminal adapter is a registration request signal. When the calling device connected to the terminal adapter inquires whether the calling-destination device is in a dial-up connection state, the VoIP calling availaiblity information on the calling destination is taken out of the database and sent to the terminal adapter through T8 to T12. The terminal adapter checks the received VoIP calling availiability information and enables VoIP calling between a communication- source calling device and the communication-destination calling device when the VoIP calling is possible, but allows calling by line connection when not.