A voice communication system comprising: a first server system being installed on a first network and including a first call control server; a second server system being installed on a second network separated from the first network and including a second call control server; and a communication line connecting the first server system to the second server system and being different from the first and second networks (3-1, 3-2), wherein the first call control server and the second call control server execute, in parallel, a call control process controlling voice communication between a plurality of communication terminals connected to the first and second networks (3-1, 3-2), a voice signal of the communication terminal in voice communication is transferred through the first and second networks (3-1, 3-2) and the communication line, a first communication terminal connectable to the first network and a second communication terminal connectable to the second network are provided as the communication terminals, the first communication terminal can access the first call control server through the first network and can access the second call control server through the first network and the communication line, the second communication terminal can access the second call control server through the second network and can access the first call control server through the second network and the communication line, out of the first call control server and the second call control server executing the call control process in parallel, one call control server operates in an active mode actually controlling voice communication, and another call control server operates in a standby mode that, when the call control server in the active mode does not operate normally, becomes the active mode in place of the call control server not operating normally, when the first network, where an active server being the call control server in the active mode is installed, is down, the second communication terminal connected to the second network accesses the active server through the communication line, and when the second network, where a standby server being the call control server in the standby mode is installed, is down, the second communication terminal connected to the second network cannot communicate while the first communication terminal connected to the first network keeps communication with the active server. a first server system being installed on a first network and including a first call control server; a second server system being installed on a second network separated from the first network and including a second call control server; and a communication line connecting the first server system to the second server system and being different from the first and second networks (3-1, 3-2), wherein the first call control server and the second call control server execute, in parallel, a call control process controlling voice communication between a plurality of communication terminals connected to the first and second networks (3-1, 3-2), a voice signal of the communication terminal in voice communication is transferred through the first and second networks (3-1, 3-2) and the communication line, a first communication terminal connectable to the first network and a second communication terminal connectable to the second network are provided as the communication terminals, the first communication terminal can access the first call control server through the first network and can access the second call control server through the first network and the communication line, the second communication terminal can access the second call control server through the second network and can access the first call control server through the second network and the communication line, out of the first call control server and the second call control server executing the call control process in parallel, one call control server operates in an active mode actually controlling voice communication, and another call control server operates in a standby mode that, when the call control server in the active mode does not operate normally, becomes the active mode in place of the call control server not operating normally, when the first network, where an active server being the call control server in the active mode is installed, is down, the second communication terminal connected to the second network accesses the active server through the communication line, and when the second network, where a standby server being the call control server in the standby mode is installed, is down, the second communication terminal connected to the second network cannot communicate while the first communication terminal connected to the first network keeps communication with the active server.