The wireless carriers' mobile-payments joint venture, after a slow start, is picking up some steam.

Isis, a partnership between AT&T, Verizon Wireless, and T-Mobile, has lined up 50 merchant partners, including heavy hitters like Coca-Cola, Macy's, and Foot Locker, as it gets closer to rolling out its service in trials in Austin, Texas, and Salt Lake City, Utah.

The merchants indicate a level of confidence in the venture's ability to roll out a widespread mobile-payment system, which would allow consumers to use specific smartphones to tap and pay for their goods and services. Beyond the big chains will be local merchants which Isis said have already been outfitted with payment terminals compatible with Isis technology.