AT&TT -0.30% is now offering cheaper monthly service if you buy or bring your own smartphones — another sign of the pressure on prices brought by a newly competitive T-MobileTMUS -0.90%.The new “Mobile Share Value” plans roll out this weekend and don’t require a contract. You can also get them once you come to the end of your two-year contract. In addition, the price of bigger data buckets have been cut.The new plans seem to have two purposes: wean subscribers off of subsidies that can run higher than $400 for the iPhone and sap carriers’ profits every time there’s a new upgrade cycle, and fight off competition from T-Mobile, whose cheaper rates have been a draw for AT&T customers coming off contract.AT&T had already experimented with backing subscribers off subsidies with its AT&T Next plan, which broke the cost of the phone up into monthly installments and allowed for more frequent upgrades. But it was criticized for effectively double charging subscribers. The monthly rates were the same as for its subsidized plans — rates that are higher because they include an embedded cost for subsidizing the phone.T-Mobile ditched service plans earlier this year and requires people to pay full price for their phone–a base model iPhone retails for $650–with the option of an interest free installment plan. T-Mobile also lets subscribers bring their own devices, and many of those customers are likely coming from AT&T, because the companies use similar network technology.Under a traditional contract plan, you buy a smartphone at a subsidized price, say $200 for the latest iPhone, and can upgrade it every two years. The carrier pays the rest of the cost of the phone. The cost of that subsidy is broken up over two years and bundled into the fee subscribers pay for monthly service.The rub is that the bundled subsidy charge doesn’t go away once the subsidy is paid off. Your monthly bills are the same, so people who don’t upgrade their phones are essentially overpaying for their service once they have paid off the phone.This was a major marketing point from T-Mobile, which argued that traditional carriers are forcing people into contracts and making them overpay. The new AT&T plans cut the monthly price by $15 for people who have their own phone, compared to people signing a contract.Under AT&T’s Mobile Share service plans, customers pay for a bucket of data and then pay a separate fee for each device attached to that bucket. The price of adding a smartphone previously depended on the amount of data you bought. Now, the cost of adding a smartphone is flat: $40 a month if you take a subsidy and sign a contract, or $25 a month if you cover the cost of your own phone. Data costs are higher for some buckets, 2 gigabytes now cost $55 a month instead of $50, but lower for larger ones like the 10 gigabyte bucket, whose monthly price falls to $100 from $120.T-Mobile added 648,000 of the most valuable wireless subscribers in the third quarter, its second quarter of adding customers after years of losses. AT&T added just 363,000 in the latest quarter, and less lucrative tablets accounted for all of them.