Isis, the mobile-payment venture backed by AT&T, Verizon Wireless and T-Mobile USA, won't debut by the end of September as planned, extending delays for a service once expected to start by the end of June.

Isis will provide an update in October on the revised timeline to introduce service in two markets, Salt Lake City and Austin, Texas, said Jaymee Johnson, head of marketing for the venture. VeriFone Systems Inc. Chief Executive Officer Doug Bergeron said in August that the project, which his company is participating in, was on track for a September start.

Isis, which will let users pay for items at stores using their mobile phones, had previously planned to roll out the service in the first half of 2012. The joint venture tweaked its strategy last year, opting to use credit-card companies to handle transactions rather than the carriers themselves, and it's taken time to ensure that payments can be made securely.

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