This contribution deals with the estimation of the relation between speech transmission quality and average call duration for a given network and portfolio of customers. It uses non-intrusive speech quality measurements on live speech calls. The basic idea behind this analysis is an expectation that the average call duration is higher for calls of a good quality since users are less disturbed during their conversation by transmission impairments. Both traditional and state of the art methods for non-intrusive speech transmission quality measurements and also for call duration monitoring are briefly described and two basic ways of the dependency analysis are shown. The relations for result uncertainty estimation are derived. A numerical example is given based on a limited training database that is constructed on the basis of real network data. Finally, potential applications are presented and discussed. The first results indicate that in the low to medium quality range (Mean Opinion Scores between 1 and 3) the call duration increases with increasing speech quality