It is not logical that when a 'Phone Call' activity being created, it shows 'completed by', even the activity is still open, or set the date for future to follow up.

Firstly is required to change the "Completed by" into "Modified by" to avoid confusion.

Then, I suggested to add a Complete and Cancel button when user created a Phone Call in the Activities section in Lead / Account / Contact / Opportunity forms. It can allow user to select "Complete" or "Cancel" once it is created.