Take your total cost and divide by the number of staff
The simple answer is to work out your total costs across the year - including all overheads - and divide this by the number of staff, in your case 25 people. This will give you the cost per member of staff.
Once you have the cost for one FTE work the average number of calls handled per year. This will then give you a cost per call. Divide the cost per call by the average handling time and this will give you a cost per minute.