T-Mobile helps parents set wireless limits.

"The service works on any T-Mobile phone and lets parents set and change limits for minutes, messages and downloads using an online tool. Once the set allowance is reached, the feature shuts off service and parents receive a notification. However, parents can still stay in contact with their child by establishing certain 'always allowed' numbers that will continue to connect even after the allowance has been spent. In addition, unlimited calling features, such as the numbers included in T-Mobile's MyFaves or in-network calling, can still be accessed."