From the TechCrunch article:
Today Jan Koum, the CEO of WhatsApp — acquired by Facebook last week for $19 billion — delivered another news bomb on top of last week’s milestone: he announced that the messaging giant is finally moving into voice — a move announced at MWC, the conference for mobile carriers that apps like WhatsApp are disrupting.
The move will put WhatsApp — and by default Facebook — more squarely in competition against the likes of KakaoTalk, Line, BBM and other messaging apps that also offer voice services.
“We use the least amount of bandwidth and we use the hell out of it,” he said. “We will focus on simplicity.” Voice will come to Android and iOS first and then following on some Nokia and BlackBerry phones, he added.
Koum today said that WhatsApp to date has 465 million monthly active users and 330 million daily users — an increase of 15 million on the number released just last week when news of the Facebook sale broke. “We couldn’t be more humbled by our growth,” he said. Interestingly, today is the company’s birthday. It was founded on February 24, 2009.
WhatsApp works across all mobile platforms, delivering messages and other content between users, but it's all keyed to user's SIM cards, phone numbers and address books. Adding low bandwidth voice services was perhaps the next logical step.
The emboldened phrase above is ambiguous in terms of its platform intentions. Common sense would dictate that he means Windows Phone and Blackberry OS 10, but WhatsApp has been old-school in the past and I wouldn't put it past the company to upgrade the older Nokia smartphone platform (Symbian) instead - this client continues to be updated regularly, even in 2014.
As does the Windows Phone client, of course. Maybe both platforms? We shall see.