Fresh from a $US19-billion ($A21-billion) takeover by Facebook, mobile messaging service WhatsApp says it will launch free voice calls by mid-year.

The introduction of free calls, revealed on the opening day of the four-day World Mobile Congress in Barcelona, would match rival application Viber’s similar offering.

“We are going to introduce voice on WhatsApp in the second quarter of this year,” WhatsApp founder Jan Koum said on Monday.

Facebook, whose 29-year-old billionaire creator Mark Zuckerberg was the star speaker on the opening day of the industry fair, announced a stock and cash purchase of WhatsApp on Wednesday.

“Five years ago we had no users, no product,” said Koum.

“Today we have 465 million active users,” he said, adding that the company still had no marketing, relying mostly on word of mouth to prosper.

WhatsApp had not planned any changes in the light of the Facebook takeover, Koum said.

“Mark really understands that for WhatsApp to stay successful it really needs to stay independent,” he said.

Viber, one of WhatsApp’s key rivals, offers free voice and messaging.

Japanese online shopping group Rakuten announced this month it would buy Viber’s Cyprus-based developer, Viber Media, for $US900 million ($A1.01 billion).