Sprint has a variety of calling plans available.
With the Sprint Fair & Flexible Plan, or the Sprint PCS Fair & Flexible America for Families Plan, your plan automatically adds low cost minutes to your plan as you need them.
Other plan types may include Sprint Free Incoming Plans or Sprint PCS Free & Clear Plans for example.
On your billing statement, look for the plan name at the top of the primary phone's individual detail page as shown below. Select your plan type, choose Individual or Shared options, then select "Go to Plan" to continue.