Nearly three years after it was announced, the joint venture in mobile payments between AT&T, T-Mobile, and Verizon will finally go national by the year's end. The consortium known as Isis said today that it would expand nationally following successful trial runs in Austin, TX, and Salt Lake City, UT. It did not give an exact time frame for the expansion, except to say it would be gradual.

Isis has been delayed as the carriers work to build a digital wallet that consumers will actually use. At stake is the future of the enormous payments market, which has become increasingly fragmented as various players scramble to make their solution the default choice of most consumers. To date, there has been no clear winner in the race to build a digital wallet — but Isis says its momentum is strong. "Over the past nine months, we have proven the power of an open platform, creating an ecosystem of literally hundreds of partners dedicated to making mobile commerce a reality," Isis CEO Michael Abbott said in a statement.

20 million US smartphones have NFC capabilities

Isis' current Android app, Isis Mobile Wallet, lets users make purchases, redeem coupons, and participate in store loyalty programs using the near-field communication technology in their smartphones. The company says 20 million US smartphones have NFC capabilities, and that support for iPhone, Windows Phone and Blackberry 10 will arrive later this year.

The user's payment credentials are stored securely in the SIM card, and that's no accident — it gives more control over the payment ecosystem to the carriers, which is largely the point of the joint venture. Today 35 devices support Isis, including the Samsung Galaxy S4, HTC One, and LG Optimus G. The company says 25 of the 100 biggest retailers nationally have installed or plan to install contactless payment terminals.

The consulting firm Aite Group estimates that by the end of the year, NFC payments will be possible at 1.3 million US locations. Isis says active users of its app make payments with it more than 10 times a month. At the same time, plenty of smartphone users still question whether phone-based payments represent any real improvement over swiping a card. It's fighting for attention in a market that among others includes the major credit card companies, Google Wallet, and upstarts like Square. Isis has the funding and the partnerships to compete in a crowded market — but whether its take on mobile payments is better than its competitors is still an open question.