AT&T is following in Verizon's footsteps by extending its upgrade eligibility program from 20 months to a full two years.

"Today, we're announcing a 24-month upgrade policy across all of AT&T's wireless products and services," the company said in a blog post.

The carrier said it wants to align its device upgrade eligibility with its standard two-year wireless agreements. The first customers impacted by the change are those whose contracts expire in March 2014 or later.

Those too antsy to wait can get a partial discount after six months if they sign a new two-year contract. Also, any device can be purchased at full retail price without a two-year agreement.

A limited-time trade-in offer is still in effect, offering $100 off the purchase of a new smartphone when customers hand in their current handset (provided the device is less than three years old and in good working condition).

Customers can check their upgrade eligibility online or by dialing *NEW# (*639#) to receive a text with the details.

In April, Verizon also tacked on four months to its eligibility cycle, which the carrier said "aligns the upgrade date with the contract end date and is consistent with how the majority of customers purchase new phones today."

A month earlier, T-Mobile announced plans to eliminate wireless contracts altogether, instead offering a $50 base plan for unlimited talk and text. Sprint still has the 20-month upgrade policy.

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