If you see that you have high percent of dropped calls probably the dial level of the campaign is too big. This means that the dialer is dialing to a lot of numbers 

and there are not enough available agents to take all the answered calls and some of them hang up.
To manage the issue, you have open the campaign that you are using, go to Auto Dial Level section, 

open the drop-down menu and lower the dial level. Don’t forget to save the changes.