You have to include the Call Duration field on the Task page layout and You can capture actual time spent on calls emails and any other activity time, with an app from the AppExchange, its called WorkIt! Automatic Time Tracking, take a look at the demo video below http://appexchange.salesforce.com/listingDetail?listingId=a0N30000001gZ0hEAE.
http://appexchange.salesforce.com/listingDetail?listingId=a0N30000001gZ0hEAE.