Can Apple Watch Series 3 LTE make phone calls when iPhone is dead?

Hi

Functions that are available on your Apple Watch when it is away from your iPhone depend on various factors including:

• Your Apple Watch model - Series 3 (GPS) vs Series 3 (GPS + Cellular):

  Apple Watch - Compare - Apple

• The features of your carrier service plan:
  • Apple Watch - Carriers - Apple
  • Make a call with Wi-Fi Calling - Apple Support
• The type of connectivity that is available to your Apple Watch at the time:

  About Bluetooth, Wi-Fi, and cellular on your Apple Watch - Apple Support

Features and requirements that apply under various circumstances are detailed here:

How to use your Apple Watch without your iPhone nearby - Apple Support