By default, all standard Salesforce pages are protected against clickjacking. Salesforce allows you to extend this protection to custom Visualforce pages. Fastcall uses an iFrames within OpenCTI. Enabling the clickjack protection beyond the default will block the Fastcall softphone from loading.
You will see the softphone utility in the footer (Lightning) but the frame is empty.
This Salesforce Trailhead gives more detail.Prevent Clickjacking Learning Objectives