Hello,
I'm going to assume (please feel free to correct me if I'm assuming wrong) that you're trying to customize the Form from the Default Solution, which in its Managed state won't allow you to customize certain elements. Additionally, the Quick View Forms for Phone Call entities have the "Customizable" option set to False, so you will need to make a copy of the one you want to edit, and disable the Managed one.
To achieve what you want:
Create a custom Solution by going to Settings --> Solutions.
Create a new Solution (you can use the Default Publisher for the Publisher field when doing this).
Once you've provided the Name, Display Name, Publisher, and a Version Number (which can be anything you like... we use a date-like format... for example, my first published build for today would be Version 2018.3.13.1), save the Solution.
Then, on the Components Section (see the left navigation area), choose "Add Existing" --> Entity.
Find the Phone Call entity in the next list, select it, and hit OK.
On the next window, select the Quick View Form that you want to edit, and click Finish.
Now, expand your Components --> Entities list on the left, and you will see Phone Call.
Expand that and click Forms, and you will see the Quick View Form.
Open it as if you were going to edit it, and click Save As...
Give it a name you will recognize, and save it.
Now, you may modify your Unmanaged copy and Disable the Managed version.
Hope this helps!
