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The authors consider a short-range cognitive network with secondary users (SUs) generating voice calls. The SUs measure the TV spectrum and classify the measured bands as either idle or occupied. When most of the measured bands are detected to be occupied, there might not be enough capacity to serve all the SUs' calls. In this study the authors identify the minimum required number of measured bands such that the probability to serve all the secondary calls is controlled. The detection performance requirements imposed by the primary systems are also satisfied. The authors formulate this problem as an optimisation problem with optimisation parameters the number of users measuring each band and the measurement time. For a small number of SUs the authors identify the minimum required number of measured bands by using exhaustive search. For a large number of users the authors propose to solve the optimisation problem by means of a greedy algorithm. The algorithm is verified against the exhaustive search results and it is found to perform remarkably well. Therefore it is suitable for practical secondary systems searching for sharing opportunities over the TV spectrum.