The Description field is shown in the Key Messages section of the Call Report page. However, even after removing Field-level Security (FLS) from the field, the section header is still displayed as a grey bar on the layout:
Is it possible to completely hide this field from the page layout?
This is working as designed. It is not possible to hide the Description field of the Key Message object even if Field Level Security (FLS) is removed. This section is hard-coded and the header is displayed regardless of the settings.
CRM Help Documentation: Using Key Messages on the Call Report