If you were on a legacy (old) prepaid plan, those codes were apparently available.
People started complaining, so T-Mobile turned them off for the newer prepaid plans [you know, that whole rule of if you had it, they don't let you lose it while you're on the plan... something Comcast apparently doesn't do].
So, if you went from PAYG to the $50 prepaid plan (I hate that they call prepaid plans and postpaid plans the same), you're going to lose any features that worked with your legacy plan.