It's a bit difficult to answer your question without seeing how the data is structured. But I assume you've tried a new calculated field: [Calls Received]/[Employees Taking Calls] ?
I think the issue might be coming from your second calculated field. You need to aggregated [Calls Taken] then return another aggregation.
Employees Taking Calls: IF SUM([Calls Taken]) > 0 THEN COUNTD([Transaction ID]) ELSE 0 END
From here, you could create your calculated field as specified above.
Hmmm I was trying it the first way: [Calls Received]/[Employees Taking Calls]
But I think I figured it out. It was returning a percentage of 350%. When I changed that to a number, it calculated correctly. Only been using Tableau for a month, so still learning.
Thanks though, as your response helped pin point my mistake.