RE: Generating a New Phone Call based on the "Next Follow Up Field" when Completing a Phone Call Activity
Hi CourtneyR11,
Are you using D365 On-premise v8?
We can use workflow to create a new phone call based on the completed phone call.
The workflow is set like this.

Select the right scope, select Record is created and Record status changes.
Add a step - Check Condition, set Phone Call, Activity Status, Equals, Completed.

Then add a step - Create Record, select create Phone Call. Select the corresponding fields.

Could you tell me where are you currently stuck?