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In communication networks that use medium bit rate speech coders, the speech signal is subjected to a destructive compression. Any other signal than voice could be highly damaged. In this paper we propose a scheme for data transmission over voice-dedicated channels using digital modulations. We will discuss the technical reasons of the degradation of the data signal introduced by the speech coder and the voice activity detectors (VAD). Then we present and compare the performances achieved by M-QAM, M-FSK and OFDM modulations for transmission over a wireless and wired networks speech coder.