There are no codes for extra minutes in the way you are thinking. Their computers generate airtime codes based on your ESN/IMEI + SIM serial; they are unique and one time use only.
Castrol EDGE 5W-30 is my drug of choice.
Toyota | let's go places
Primary: iPhone 5S and LG G2: 5 lines, 1x $15 4.5GB data, 2x $5 Android web, 1x $2.99 T-Zones, myFaves, corp discount + $10/mo loyalty discount = $88/mo tax inclusive
Secondary: Optimus Dynamic II: TracFone (Verizon); Galaxy Epic: RingPlus (Sprint)
Tablet: iPad Mini Retina w/ T-Mobile 4G LTE | Galaxy Tab 3 w/ T-Mobile 4G LTE