On Call pay for Dispatcher Position

Alberta Employer in the trucking industry.  We have our dispatchers work a 7 day on, 7 day off shift during which they are responsible for all dispatching trucks and responding to driver’s.  It is very tedious and difficult to determine the amount of “overtime” they would have after leaving the working place and taking calls at home as some nights they may get no calls and other nights several calls sporadically spaced through out the night.
We were wondering if it might be possible to pay them a salary for the hours spent at the work place and then implement an On call premium to compensate them for having to respond to calls once at home and have this amount compensate them for any overtime i.e. answering of calls that may required during their shift.