The disclosed embodiments provide a method and apparatus for interoperability between CTX and DTX communications systems during transmissions of silence or background noise [FIG. 2]. Continuous eighth rate encoded noise frames are translated to discontinuous SID frames for transmission to DTX systems (402–410). Discontinuous SID frames are translated to continuous eighth rate encoded noise frames for decoding by a CTX system (602–606). Applications of CTX to DTX interoperability comprise CDMA and GSM interoperability (narrowband voice transmission systems), CDMA next generation vocoder (The Selectable Mode Vocoder) interoperability with the new ITU-T 4 kbps vocoder operating in DTX-mode for Voice Over IP applications, future voice transmission systems that have a common speech encoder/decoder but operate in differing CTX or DTX modes during speech non-activity, and CDMA wideband voice transmission system interoperability with other wideband voice transmission systems with common wideband vocoders but with different modes of operation (DTX or CTX) during voice non-activity.