Transcoding needs and selection are performed by a central call control function (17, 35). Each transcoder (18, 23, 34, 36) registers with the appropriate call control (17, 35). The call control then determines whether a vocoder is required at all. If vocoding is required, call control (17, 35) selects the appropriate transcoder (18, 34) and inserts the appropriate transcoder into the bearer traffic stream (16).