I feel for you. Unfortunately, I don't know if I've ever seen T-Mobile advertise expiration date alerts as a feature and if it ever was, I'm sure it wasn't guaranteed. That customer service message is either for postpaid (which is not 24/7) or outdated prior to prepaid CS being outsourced.
It does come down to the customer remembering to refill on their own. Boost Mobile is excellent at sending reminder text messages to refill, I think at 20 days, 8 days, and 2 days. Better yet is TracFone where the due date is right on the screen and the expiration dates stack; so If I have 25 days left and add a new card which gives me 90 days, I can add it now and get 25 + 90 days = 115 days instead of 90 days from today. It takes away the "I'll wait til a day or two before my balance expires to get the most for my money" kind of thing.
Castrol EDGE 5W-30 is my drug of choice.
Toyota | moving forward
Primary: Galaxy Note II & iPhone 5 (AWS): 5 lines, 1x $15 4.5GB data, 3x $2.99 T-Zones (truly unlimited 4G data), myFaves, corp discount + $10/mo loyalty discount = $88/mo tax inclusive
Secondary: iPhone 3GS: Airvoice Wireless, Samsung Fascinate & HTC Trophy: Page Plus