An uplink transmission frame transmitted from a base transceiver station to a transcoder rate adaptation unit in a cellular mobile radio network includes in the conventional way an information area in the form of at least one data field and at least one control field, the latter including a subfield representative of the validity of data transmitted in the at least one data field. This subfield is substantially at the start of the information area. This frame structure prevents transmission of invalid speech signals during a call from the cellular network to a fixed network such as the public switched telephone network at the time of handover affecting a mobile station involved in the call.