In this paper we propose a wideband speech encoding scheme (50-7000 Hz) having a bit rate of 12.3 kbit/s which could be used for the GSM Fullrate channel. The coding scheme is based on 2 unequal subbands from 0-6 kHz and from 6-7 kHz. This approach was motivated by experimental evaluation of the instantaneous signal bandwidth of speech frames. The lower subband is encoded using code-excited linear prediction (CELP). The higher subband is replaced at the receiver by aliased components of the lower band using an interpolation filter with a cut-off frequency of 7 kHz. By informal listening tests the speech quality was rated higher than the speech quality of the CCITT G.722 wideband codec operating at 48 kbit/s. In comparison to the GSM Fullrate codec with 13 kbit/s, naturalness and intelligibility are improved significantly.