To a great extent the number of calls you get depends on how frequently you answer the phone and can say 'yes'. There are some companies that seem to endlessly call with no bonus available. Break them of the habit. The shops they are calling for there is a 95% chance you have seen already on the board and are not interested in it at the price offered or you would have taken it. Nobody else obviously is interested in it at that price either. You don't owe a scheduler helping them out unless they are taking care of you.
If you can turn it into a horse trade, fine: "I will do A for you, but I want B assigned to me for next month. I have been requesting it for months and am never given it." If they agree, the chances are they can do nothing about it at the moment, but make sure you have their email addy and remind them next month. If they don't follow through to get B assigned to you, just don't help them out in the future.