The Isis wallet - owned by AT&T Mobility, T-Mobile USA and Verizon Wireless - launched in November. It lets Americans with an NFC-enabled Android handset, a special SIM and an app make contactless payments at more than 200,000 merchant locations.

The wallet is now supported on 68 smarthphones and comes pre-loaded on 14 devices. In addition, earlier this year it came to the NFC-less iPhone, although customers have to pay for a specially-designed sleeve for their handset.

With a rapidly expanding base of compatible handsets and merchants helping to push the service through special offers, wallet activations have averaged over 20,000 a day in the last four weeks, with the growth rate doubling over the prior month.

Jamba Juice says that it has seen strong growth in the number of people using Isis in its stores. By the end of the first quarter, its customers had redeemed more than 270,000 free smoothies by using the wallet.

Isis is now working to bring in more partners to entice users with deals similar to the Jamba Juice giveaway as it seeks to win out in the fast-growing, lucrative mobile contactless payments market.

However, competition is set to increase thanks to the rise of Host Card Emulation. Until recently telcos have held a trump card in mobile NFC payments through their control of the Secure Element. By studiously ignoring Google Wallet the Isis partners strangled adoption of the search giant's rival.

But Google, alongside Visa and MasterCard, is now throwing its weight behind HCE as a means of escaping the need for telco help, opening the way for it to offer a stronger challenge to Isis.

Nor is the way any clearer on the merchant front, with many of the nation's largest chains forming their own mobile payments consortium, dubbed MCX.