A communication system is provided that includes a network node, a first terminal which supports an EVS (Enhanced Voice Service) codec, and a second terminal. The EVS codec supports an AMR-WB (Adaptive Multi Rate Wideband) codec compatible mode and an AMR-WB codec non-compatible mode, the AMR-WB codec having a dedicated payload format, where the AMR-WB codec compatible mode is compatible with an AMR-WB codec and the AMR-WB codec non-compatible mode is non-compatible with the AMR-WB codec. The first terminal and the second terminal negotiate the AMR-WB codec non-compatible mode in a session negotiation to select the AMR-WB codec non-compatible mode to be used for the communication between the first terminal and the second terminal when the communication starts.