AT&T is reducing cellphone rates for family plans with large amounts of data. The new rates are available starting Sunday and apply to plans with at least 10 gigabytes of data shared by two or more phones on a single account. Savings start at $10 per phone per month. A family of four sharing 10 gigabytes will pay $160 a month combined instead of $200 under the most recent rates. The changes represent AT&T's latest effort to get customers to pay more overall by upgrading from lower-use plans.