Verizon Wireless will let its customers make free or low-cost calls by using Skype on its smart phones, making it the first major U.S. operator to strike a marketing agreement with the Internet phone company service.

While operators such as AT&T Inc tolerate customer use of Skype applications on cellphones, Verizon is the first U.S. operator to actively push the service, which will work on devices such as BlackBerry's Research In Motion Ltd.

Verizon customers with data service plans will be able to use the Internet telephony service on nine different smart phone models starting next month, Verizon and Skype said at the Mobile World Congress in Barcelona on Tuesday.

The companies said they had created a Skype mobile offering fro 3G smart phones allowing users to make unlimited Skype-to-Skype calls on the U.S. carrier's network as well as international calls on Skype calling rates.

(Reporting by Nicola Leske in Frankfurt)