It's currently not possible to display the average number of calls per day with the reporting tool. The average would have to be calculated prior to adding the properties to the report using a calculation property.
Thanks for reaching out! Averages can be applied to eligible metrics (numeric fields) in the custom report builder. For # of sales activities, we can only display them by distinct count. The outcome would be a report that displays the total number of calls per day per user in this case.