The 'New Phone Call' button is intended to be used when a phone call needs to be scheduled for the future. It allows the creator to set a date for the phone call, and it is optional for the creator to reserve time in their calendar and create reminder alerts. By default the status of the new phone call is set to Scheduled.
The 'Log Phone Call' button is intended to be used when the creator is currently on the phone, or just completed the phone call. It does not allow the creator to set the date on which this phone call occurred, but automatically sets this as the day on which the phone call record is created. It is not possible for the creator of the phone call to set a reminder alert for the phone call. By default the status of the phone call is set to Completed.
By using the two different buttons a user who is on the phone, or just recently completed their call can speed up their logging of the call without seeing unnecessary fields.