Isis is upping the ante in its bid for new users. The mobile payment joint venture between AT&T, Verizon Wireless and T-Mobile is now offering active Isis mobile wallet users a $100 Amazon gift card if they enable a credit card in the mobile wallet. (Droid Life has a screenshot of the email Isis sent its users.)

The offer may have limited uptake; currently, users can add cards from only a few issuers, including American Express and Chase. It's also available only to wireless subscribers who are in one of Isis' two pilot cities (Salt Lake City, Utah, and Austin, Texas), or who have ordered a special NFC-enabled SIM card from one of the partner carriers.

When pilot programs for Isis were launched late last year, customers were encouraged to participate with a $25 bonus that was added to their Isis prepaid account.

Isis hasn't released any information about the number on participants in its pilot programs. Sources familiar with Isis' efforts to this point have pegged investment in the consortium from the JV's three owners at around $250 million since its launch nearly three years ago. 

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