First, it was free text messages. Now WhatsApp users can call each other - provided they have an Internet connection - for free.
Users simply click on the "Calls" tab to phone a friend, who can be located anywhere around the world. But this works only when both parties are on the latest version of the app and have an Internet connection: Wi-Fi or mobile data.
With the new feature, WhatsApp joins the ranks of competing apps like Viber, Skype and Apple FaceTime - services which have been eating into telcos' traditional business revenue, including voice calls and SMS.
But Singtel told The Straits Times that it will take on such competition, saying it is reviving plans to launch a WhatsApp and Skype equivalent offering both Internet voice call and text messaging services. No details are available pending the app's launch, which will be "soon".
In early 2013, Singtel came close to launching its own version of WhatsApp and Viber, called LoopMe, until a legal tussle broke out between two software firms that had worked on the app. They are Singapore-based Amdocs and Israel-based Tjat. Singtel then abandoned the app for undisclosed reasons.
Senior analyst Clement Teo of United States-based market research firm Forrester said that free calls and text messaging is a double-edged sword for telcos.
"In the short term, telcos' mobile voice roaming revenue may still be affected," Mr Teo said.
Conversely, such calls may end up being data hungry, which could benefit telcos.
"The upside is users may consume more mobile data. You need a good connection to make a voice call, and Wi-Fi connections are not available everywhere," he added.
M1 said that Internet voice services operate on a "best effort basis", whereas traditional voice services offer better quality.
StarHub said that it still has a "sizeable base" of customers who require high-quality voice connections.
The call feature has been released to Android users first. According to market research firm GfK Asia, Android users account for more than three-quarters of the smartphone users here.
US-based Internet statistics firm Statista estimates that more than 70 per cent of all mobile Internet users in Singapore are on WhatsApp. Globally, WhatsApp's users number more than 700 million.
An iPhone version of the app is expected to be available "in a couple of weeks", according to several online reports. WhatsApp, which was acquired by Facebook last year for US$19 billion (S$26 billion), did not respond to queries from The Straits Times by press time.
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