In the digital mobile radio system GSM (Global System for Mobile Communications) there is a need for reducing the subjective effects of residual bit errors by error concealment techniques. Due to the fact that the standard does not specify these algorithms bit exactly, there is room for new solutions to improve the decoding process. This contribution presents a new approach for optimum estimation of speech codec parameters applied to the GSM system. It requires a bit reliability information provided by the channel decoder (e.g. soft-output Viterbi algorithm -- SOVA in GSM) and additionally exploits residual source redundancy, i.e. some a priori knowledge about codec parameters. The proposed method includes an inherent muting mechanism leading to a graceful degradation of speech quality in case of adverse transmission conditions. If the channel is error free, bit exactness as required by the GSM standard is preserved.