If you are talking about theoretical maximum calls a day then I agree with what has been said. To get nearer to the real number you need to separate working hours from staffing hours e.g. 8 hours clocked time minus half hour lunch and two half hour breaks means an effective working time of 7 hours.
You also need to factor in an allowance for pit stops, training, coaching, team leader time as well as shrinkage for sickness and holidays.
Finally you haven't reall made an allowance for the calls being offered. Your assumptions are based on having a team which is either exactly the right size for the demand or too small so that when one call ends another is available to be answered. You would need to use a modelling tool to get a feel for the impact of this. Although we have Totalview for a lot of our modeling there are some good low cost modelling tools. One I use for playing with capacity planning is Kooltoolz but there are others out there