V.14

In V.14 the only activity in which is possible to add a time and reminders is the meeting activity due that when you schedule this activity the calendar app is open.

A alternative to add time and reminders when scheduling calls is to change the configuration of the Call activity. By going into activity types (in the CRM app) and editing the activity named "call". 

What you need to change is the "action to perform" and select "meeting". This way, any time you schedule a call, the calendar app will open letting you to chose a specific time, duration and creating reminders.

Hope this helps you and with the new version that is coming on October more features will be available.