12 would be the constant. 
This is because:
 1. it doesn't change regardless of minutes used. If you use 1,000 minutes or if you uses 3 minutes you will still have to pay those $12 therefore it is a consistent 
2. because it fulfills the requirement to be a "constant". The requirement is that it is a number without any variables attached to it. It is standing on its own. 
Hope this helped!
~Just a girl in love with Shawn Mendes