Positive Networks is using phones as a way to authenticate users as they log into corporate networks -- and offering the service for free.

At Interop Las Vegas on Monday, the company said the feature, called Phone Factor, dials up a designated phone number when users try to log in. Users key in a particular number or letter or a PIN. If they do so successfully, Phone Factor triggers access to the network or application they are trying to use.

The service is an alternative to other two-factor authentication schemes that require something the user has and something the use knows. In this case the user has a phone with a specific phone number and knows the associated PIN. In other cases, it involves security tokens that require infrastructure and maintenance.

While the service is free, Positive hopes to make money on it by charging for a range of add-ons, such as support, reporting and speech recognition.

Phone Factor can be used to log in to a variety of enterprise applications including Citrix, Outlook Web Access, SAP and Windows/Novell logins. It also works with remote access gear from Aventail, Cisco, F5, Juniper and SonicWall. Phone Factor is compatible with Web applications IIS, Apache (in conjunction with RADIUS) and BEA Weblogic.