The probability density function of PCM encoded speech signals has been measured, and it has been found that, as a result of companding, the distribution is rather flat except in the neighborhood of the zero level. On the basis of the measured probability density function (PDF) a variable length prefix code was developed that allows an increase in the traffic capacity of PCM transmission systems. The benefit obtained is mainly due to the high probability of the zero level. According to the proposed coding method, inactive channels are maintained by sending only one code bit per frame; they thus can be activated without delay and without signaling. This is an advantage with respect to the time assignment speech interpolation (TASI) system [1].In the paper the capability of this method has been investigated.