If your account has the Projects module active, in the calendar's week view you'll see a bar below each day of the week. This shows your total availability for that day, based on the amount of hours per week you have set. The bar will fill up based on the tasks and meetings you have planned for that day. Calls are too unpredictable to have a fixed timeframe, so they are not taken into account.
When the bar shows green, it means everything's okay and you have some room left in your schedule. Orange means you're already overbooked for that day in particular.
The availability bars also take all planned tasks and meetings from your projects into account. So even if a task is not exactly scheduled in your calendar, but it is due on that day and planned via the project planning, your availability decreases.