A communication system comprising: a network node ; a first terminal which supports an EVS (Enhanced Voice Service) codec and an AMR-WB (Adaptive Multi Rate Wideband) codec; and a terminal which supports the EVS codec, wherein: tthe EVS codec supports a compatible mode and a non-compatible mode, the compatible mode being compatible with the AMR-WB codec, and the non-compatible mode being non-compatible with the AMR-WB codec and not capable of being used as the AMR-WB codec, the AMR-WB codec being the legacy codec, the first terminal and the terminal are adapted to negotiate the non-compatible mode in a session negotiation for communicating data between the first terminal and the terminal when said communication starts in a packet switching, PS, network, the network node that is adapted to detect if the negotiated non-compatible mode of the first terminal is switched to the compatible mode or the AMR-WB codec in communicating data between the first terminal and the terminal, and upon such a detection, to transmit a signal for the terminal to switch the negotiated non-compatible mode of the terminal to the compatible mode by using a payload format of the EVS codec; and the terminal is adapted to switch the negotiated non-compatible mode of the terminal to the compatible mode without changing the EVS codec, based on the signal. a network node ; a first terminal which supports an EVS (Enhanced Voice Service) codec and an AMR-WB (Adaptive Multi Rate Wideband) codec; and a terminal which supports the EVS codec, wherein: tthe EVS codec supports a compatible mode and a non-compatible mode, the compatible mode being compatible with the AMR-WB codec, and the non-compatible mode being non-compatible with the AMR-WB codec and not capable of being used as the AMR-WB codec, the AMR-WB codec being the legacy codec, the first terminal and the terminal are adapted to negotiate the non-compatible mode in a session negotiation for communicating data between the first terminal and the terminal when said communication starts in a packet switching, PS, network, the network node that is adapted to detect if the negotiated non-compatible mode of the first terminal is switched to the compatible mode or the AMR-WB codec in communicating data between the first terminal and the terminal, and upon such a detection, to transmit a signal for the terminal to switch the negotiated non-compatible mode of the terminal to the compatible mode by using a payload format of the EVS codec; and the terminal is adapted to switch the negotiated non-compatible mode of the terminal to the compatible mode without changing the EVS codec, based on the signal. tthe EVS codec supports a compatible mode and a non-compatible mode, the compatible mode being compatible with the AMR-WB codec, and the non-compatible mode being non-compatible with the AMR-WB codec and not capable of being used as the AMR-WB codec, the AMR-WB codec being the legacy codec, the first terminal and the terminal are adapted to negotiate the non-compatible mode in a session negotiation for communicating data between the first terminal and the terminal when said communication starts in a packet switching, PS, network, the network node that is adapted to detect if the negotiated non-compatible mode of the first terminal is switched to the compatible mode or the AMR-WB codec in communicating data between the first terminal and the terminal, and upon such a detection, to transmit a signal for the terminal to switch the negotiated non-compatible mode of the terminal to the compatible mode by using a payload format of the EVS codec; and the terminal is adapted to switch the negotiated non-compatible mode of the terminal to the compatible mode without changing the EVS codec, based on the signal. <EOS>