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:

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.