Facebook teamed up with mobile-payment start-up Zong to test mobile-phone payment for Facebook Credits, according to Silicon Alley Insider.

Users of the social-networking site are offered the mobile payment option and, should they choose it, they enter their mobile numbers and receive a confirmation PIN via text message, Silicon Alley Insider reported. When the PIN is entered in the gift-shop checkout confirmation, the Facebook Credits are billed to the user’s account with the mobile carrier.

A Facebook spokesman told Inside Facebook:

We are looking at extending our virtual currency, Facebook Credits, via a very small alpha test with a handful of developers. As part of this test, we are working with Zong to explore ways for users to easily purchase credits and virtual goods by entering their mobile number, rather than credit-card information, in the supported application. It’s also available as a payment option in the Gift Shop for a small group of users.

And Zong vice president of product and marketing Hill Ferguson told Inside Facebook:

As Facebook identifies ways to incorporate Facebook Credits into third-party applications and help developers monetize their applications, we are testing an integration that would allow users to purchase Facebook Credits through our mobile-payments service.