Assigning trunks to phones

I am using asterisk with freepbx in which E1 card is placed.

    I am using Zaptel hardware trunks. I want to assign a trunk to a particular extension as a dedicated trunk such that no one can use this trunk. 
       I have written a dialplan such that when a phone makes a call it is passed through any of the trunks which ever is free. How can i achieve the above said dedicated trunk.

Please help me through the GUI freepbx also.

Thanks in advance.