PURPOSE: To improve availability of a trunk circuit as well as the number of call connections by measuring periodically the using conditions of the trunk circuit or a subscriber circuit and deciding the circuit transfer enable capability of these circuits based on the results of said periodical measurements.

CONSTITUTION: An exchange 2 counts periodically the number of data transfer packets to be sent for each trunk circuit 1 and at the same time calculates the transfer enable throughput at the relevant time point of the circuit 1 from those count results of the exchange 2. Then the state transition probability is calculated for the transfer enable throughput group which shifts every cycle and a state probability transition matrix is stored. In a call setting mode it is decided whether the relevant call should be accepted or not according to the transfer enable throughput value, the presumed throughput value displayed by a subscriber and said transition matrix. Thus the load is reduced with both a terminal and the exchange 2 and it is possible to improve availability of the circuit 1 as well as the number of call connections together with suppression of occurrence of congestion.