Why is there a limitation on the maximum number of departments that can be added at a single level IVR?

Why is there a limitation on the maximum number of departments that can be added at a single level IVR?

We cannot add more than 9 departments at a single level in an IVR.

This is done to:
  1. Make it easy for customers to enter their input as all phone keypads consist of only 9 digits.
  2. Differentiate between user input and department input. The user input count starts after 10, so if you assign 10 departments you wouldn’t be able to identify if it was a user input or a department input.