just a few ideas,
You can predict the volume that will come in. There are many applied math. procedures to predict the number of incoming calls. e.i. Arma, Holt-Winters, seasonal HW, etc. But you always have to make caleder adjustments.
You need 3 years of data to have a workable medium for the calender adjustments. You can use this data in order to improve your prediction model. Try past data in order to see what the result is yoy model (use the best fit method in order to diagnose the model. You can have more models at the same time. You have to weight the models. ax+by+cz, and the factor must be able to change ongoing.
I can tell you more but my battery is low.
regards jan kalden