A call slot will usually have start and end times as specified by the calltype for that day. It may have a different time, in which case a time marker will appear next to the users name.
You can change the times of a call slot, or add a note, by:
1. pressing edit from the top right of the call screen
2. selecting the "edit" button next to the call slot
3. A dialog will appear which will allow you to change the times, add a note, and also change the user. Press save when you are done.