By Tarmo Virki

PARIS (Reuters) - Instead of just "liking" - why don't you call?

A simple proposition like this could lift Facebook's annual revenue by $800 million and give the social network a direct-billing relation with millions of users, the chief of mobile calling firm Rebtel told Reuters.

Andreas Bernstrom said a Facebook voice service charging for calls going into traditional telephones could in theory create the world's second largest telecoms network by customers, after China Mobile and ahead of Vodafone.

"A serious step from Facebook could have a significant effect on operators," Bernstrom said.

The CEO of Rebtel, the world's second largest Internet calling app after Skype, said his calculations are based on Skype-like pricing and uptake of the service, that would give Facebook more than 8 million credit cards.

Europe's traditional phone companies are facing intensifying competition from Internet-based services like Microsoft Corp's Skype, Rebtel, Viber and Tango.

"Facebook has great opportunity to build an app for calling functionality in much the same way they have one for messaging and photos," said Bernstrom, who added that he is not currently in talks to sell his service to Facebook.

"Not only does this generate a very large revenue potential ..., but it also allows Facebook to start building a giant database of credit cards, which will be hugely important for future cross selling opportunities."

(Reporting By Tarmo Virki; Editing by Tim Dobbyn)