Abstract: A codec is introduced, according to a new speech encoding method, into a speech transmitting transceiver of a digital telecommunications system in order to use a "new" codec and an "old" codec in parallel in the system. A codec is selected by implementing a handshaking procedure between transceivers, handshaking, where a speech encoding method implemented in all transceivers and previously used in the telecommunications system concerned, is used at the beginning of each connection. At the beginning of a phone call and after handover, the method checks whether both parties can also use the new speech encoding. The handshaking messages have been selected so that their effect on the quality of speech is minimal, and yet so that the probability of identifying the messages is maximal.