Isis, a joint venture among Verizon Wireless, AT&T, and T-Mobile USA, will likely start trials of its mobile-payment service in August, according to a person familiar with the launch plans.

The August target, however, isn't set in stone. CNET has gotten indications elsewhere that the timing may change. A representative for Isis only said the service would begin trials in Salt Lake City and Austin, Texas, this summer, pushed back from the prior forecast of a mid-year launch.

Isis will join Google Wallet in its attempt to get people to start paying at cash registers with their smartphones instead of a credit card or cash. Unlike Google's initiative, which launched last year amid a lot of fan fare, Isis has taken a slower approach, building up a wider number of partners, including retailers, payment networks, and banks, in an effort to offer consumers more options when signing up.