I made a phone call. How to delete it from a list?

You should not delete any record from the system, because every datum is important. Completed events and tasks should have status held (for events) and completed (for tasks). After a while we tend to forget all Activities referring to a certain Account or Opportunity therefore Acctivity's history is very useful.

After clicking on Save button a new window opens.

Select some other Activity or click on No button.