Simon (or Maria, if you have chosen her) may stop calling if he/she has already assigned you a non-quest-related project. You should have a project listed in your To-Do List (checkmark icon in the bottom right), and there should be an open studio location waiting for you.
If you are still not receiving calls, please check that:
- You have updated to the latest version of Kim Kardashian Hollywood
- You are not already at the current maximum level
If you are at the maximum level, you may have finished all the available quests. With every update, more levels are added, which will trigger more quests.