A communication system comprising: a network node; a first terminal which supports a first codec and a second codec; and a terminal which supports the first codec and another codec, wherein: the first codec being a codec different from a legacy codec, the first codec supports a compatible mode and a non-compatible mode, the compatible mode being compatible with the second codec and capable of being used as the second codec, and the non-compatible mode being non-compatible with the second codec and not capable of being used as the second codec, the second 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 second 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 first codec; and the terminal is adapted to switch the negotiated non-compatible mode of the terminal to the compatible mode without changing the first codec, based on the signal. a network node; a first terminal which supports a first codec and a second codec; and a terminal which supports the first codec and another codec, wherein: the first codec being a codec different from a legacy codec, the first codec supports a compatible mode and a non-compatible mode, the compatible mode being compatible with the second codec and capable of being used as the second codec, and the non-compatible mode being non-compatible with the second codec and not capable of being used as the second codec, the second 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 second 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 first codec; and the terminal is adapted to switch the negotiated non-compatible mode of the terminal to the compatible mode without changing the first codec, based on the signal. the first codec being a codec different from a legacy codec, the first codec supports a compatible mode and a non-compatible mode, the compatible mode being compatible with the second codec and capable of being used as the second codec, and the non-compatible mode being non-compatible with the second codec and not capable of being used as the second codec, the second 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 second 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 first codec; and the terminal is adapted to switch the negotiated non-compatible mode of the terminal to the compatible mode without changing the first codec, based on the signal.