State of balance money / included minutes from mobile operator interest almost everyone. Few exceptions: unlimited calling to fixed monthly fee and credit TA-paid communication services. You can, of course, keep the account a couple of hundred USD and do not bother cost control, but for most of us is a "solution" is unacceptable. Yes, and the deposit interest operators do not pay, although it could do something like progressive discounts depending on the amount of down payment. The first attempt of this kind can be considered a 15 percent bonus on every second installment in Bee +, but this is a topic for another conversation.