there are a range of possibilities, therefore we need an inequality and we also need a better problem statement because this leaves out a great detail of necessary information

we assume the problem wants to know how many calls per month a particular employee might receive

we also assume that all calls pertain to timeshares

we also assume that there is no way to differentiate between calls received by regular employees, internet employees and the supervisor... well, ok, we ignore the supervisor as not an employee

13+5 = 18 employees

it's irrelevant what hours the call center operates because we assume each of the employees must work every hour the call center is open - if not, then we assume that every employee has an equal chance of receiving any particular call

let 'x' represent the number of calls per month an employee might be expected to receive:

30000 / 18 <= x <= 40000/18

1667 < x < 2222

any given employee could be expected to handle anywhere from 1,667 to 2,222 calls per month