I wouldn't call in and let them call you and then don't answer the phone and show up to work at your normal 1730 start time for the local sort. Then quote the part of the contract that BigUnionGuy posted above.
Currently you playing there game and until you stand up for your contractual rights they will walk all over you as they currently do.
The only time I mainly go on call is during the summer. The reason is that cover drivers bump back to local sort, putting me on call. Is this acceptable? or greiveable?
I understand your frustration, but is this an issue you really want to fight. If you plan on being a full time driver, do you want management to remember how much of a PITA you are to them. If you don't want FT driver then raise hell.