To restore Activity Log Fields which were deleted, follow these steps:
Step 1 Log into CRM
Step 2 On the landing page, click on Administration from the main menu
Step 3 Hover the mouse over Setup, and click on CRM Settings
Step 4 If the active tab is not Activity Log Fields, click on it within the list
Step 5 To restore a field marked for deletion, click on the field in the list
5.1 Click the Restore button
Article Reference: CLTX504