OTT messaging specialist WhatsApp plans to add voice capability to its offering in the next few months, company CEO Jan Koum announced on Monday.

"We're going to introduce voice on WhatsApp in the second quarter of this year," Koum said during a conference session at Mobile World Congress, receiving a round of applause from the audience.

Voice calling will first be available for Android and iOS users, with support for the Windows and BlackBerry platforms following at a later date.

"[We have built] the best voice product out there. We use the least amount of bandwidth and we optimise the hell out of it," Koum said.

While some operators insist they have nothing to fear from WhatsApp straying further into their territory, others still have significant voice revenues to protect.

"This is exactly what we have been planning," said Tele2 CEO Mats Granryd. "They make people use my network so I can charge them for data," he said.

"In many of our countries we charge only for data," Granryd explained. Customers get voice and SMS for free. "Sorry Mr WhatsApp, but it's free in the Tele2 world."

But Granryd's company, which operates in 10 markets in Europe and Kazakhstan, is in a different position than Millicom, an emerging markets operator with a presence in Latin America and Africa.

"Take it easy," Millicom chief executive Hans-Holger Albrecht urged Koum. "70% of our revenues [come from] voice."