Hi Abdel,
In nokia system the thing you are saying is implemented using the parameter TR (Trunk Reservation). By enabling this feature you can reserve the radio channel for preority class user. There are two methods to implement this :
1) Static method: In this a channel is permanently reserved for the preority class user. No call can go on that channel except that preority user.
2)Dyanamic method: In this no dedicated channel is allocated but channel is made empty by dropping the call on that channel if call is going on. The preority class user will never come to know that there is any congestion in network.
Now these preority classes of user are defined when your SIM is got activated in HLR. Your SIM preority class you can check from HLR.