Formula Help - Average Productivity Per Day

You'll need to break out some kind of 'Schedule' table in your app that can join the Person with each day of the month. The idea with that table is it can act like a container for each person - and bind/summarize their activity for that particular day. You wouldn't necessarily have to manually schedule it - you could evaluate if they had ANY calls that day and treat that like a day you would consider for reporting and then evaluate productivity accordingly. 

The setup would be a new table with essentially two fields to start - Related Person (if you have a table of people you would relate it to that table and add whatever lookup fields help) and then a field for 'Day' which would be a single date field. From there - you can either change the primary key to a string field that concatenates person with the date as a unique string, and then relate your new table to Calls, OR you can use Formula queries to do the work that a the relationship in my comment above would be doing to summarize the data. 

The rationale is that you need to isolate individual fields for each day for that person to determine 1) if they were active and 2) evaluate their activity. This kind of setup allows you to do that and then some. 

The above also keeps it all on a daily basis and then allows you to aggregate / report monthly. You could also pivot what I suggested, and instead do Person Months table - where instead of Days you isolate the month as a whole. You could then count the number of unique days they worked and the total calls and do your average. You lose some of the visibility in day over day trends this way since its now the month as a whole, but the end result is still the same.