To unlock your Alcatel 4043S, you typically:
If you do not meet the carrier's requirements, you can purchase an unlock code from a reputable third-party service.
with a carrier other than the one it was originally sold for (e.g., switching from AT&T or Lively/GreatCall), you need a Network Unlock Code
This is a complete guide to unlocking the (also known as the Alcatel 1S or Alcatel 3V depending on the specific regional variant).