It currently is not possible to display the associated contacts in a task or phone call search. Enhancement 207983 has been logged for this feature request. However, the following workaround can be used.
1. Navigate to Reports > Pipeline Analysis > Sales Activity By Customer > Customize detail.
2. Edit Columns section > Highlight Customer column and click Remove Column button.
3. Add Fields window > Expand Sales Activity folder > Expand Contact folder > select Name.
4. Filters tab/section > Add Fields window > Expand Sales Activity folder > Select Activity Type and set it equal to Task or Phone Call.
5. Click Preview.
The report should display tasks or phone calls with all associated contacts. Users can then return to the report customization window and customize further if required and save.