That's a decent way to calculate things, but don't forget about documentation time, the few seconds between calls before the next one comes, etc. You are also assuming no breaks of any type and that your cloud is distributing calls back to back without delay. Make sure your average call time is as accurate as possible to get a better idea of utilization rates.
A person taking 1000 calls over 22 working days in a month is taking 45.45 calls per day. At 5 minutes for each call, they spend 227.27 minutes, or 3.78 hours on calls throughout the day. So, 4.21 hours of the day are spent doing...? But again, this is based on 1000 being correct, 22 working days being the average (what about sick/vacation days?), and 5 minutes per call being correct. That's a lot of variables that need to be scrutinized before accepting that 4.21 hours per day are wasted by each employee.
I would vehemently encourage that you check the underlying assumptions before drawing any conclusions.