This question asks for a specific miscalculation of how large chance that I get at least one call every day of the week if I in total get 12 calls during a week.
If we instead consider the question how to calculate it...
For some reason I think there should exist a number theoretic solution to this. We have the concept of partitions which count the number of ways to sum something up. For example 5 can be written as $5=4+1$ or $5=3+2$, $5=3+1+1$, in total there are 7 such ways to sum, called partitions.
If we place 7 calls over 12 days then we have 5 calls left to place. How can we place these remaining 5 calls over 7 days? Placing 5 calls over 5 days, then partition could help us, but not here. It feels like we should be able to use partitions or some generalization of them to express this.