The invention provides a method, a communication system, a communication network, a communication apparatus and a communication terminal enabling the voice coding mode during voice communication to be changed by a common operation independently of diverse network configurations. A first network monitors the communication situation including quality of a radio link during voice communication of a first terminal, and determines an appropriate coding mode based on the communication situation. In addition, the first network sends to the first terminal a first request for change to the appropriate coding mode. Upon reception of the first request, the first terminal sends a second request for change of the coding mode to a second network. Upon reception of the second request, the second network changes the coding mode of the voice codec given to a second terminal engaged in voice communication with the first terminal, and sends back to the first terminal a response to the second request. The first terminal, which receives the response to the second request, sends back to the first network a response to the first request.