PRMA (Packet Reservation Multiple Access) represents a class of packet-contention access protocols for wireless microcellular networks that handles both voice and data traffic. We consider a dynamic control procedure for the operation of PRMA systems and analyze it by means of a Markov model of the PRMA systems. The model gives indications of some margins for performance improvement under the hypothesis that the number of contending terminals in the system is exactly known. Since this last value is not available in general, we propose an estimator of its value and simulate the dynamic control based on such estimation. The results show that the estimation error can cause the degradation of performance, this effect is particularly evident in the case of PRMA++ protocol.