Mobile messaging firm Kakao Talk has launched its mobile VoIP service for users in its home country South Korea, despite concern from mobile carriers that fear it will overload their data networks and steal voice revenue.

The company has begun trialling the free call feature in beta in Korea, where it has 36 million users and says it is installed on 98 percent of smartphones. The introduction comes after it launched the feature for international users two weeks ago.

The app’s call service is initially available for iPhone users in Korea, with the voice trial set to hit Android-powered phones this week. As is the case for Kakao Talk’s 9.2 million international users, the BlackBerry app will not support calls.

Unsurprisingly, the move has not been welcomed by Korean operators who fear the additional usage will put significant strain on their data network, according to The Chosunilbo. Though the online newspaper does not include any direct quotes or refer to specific operators.

Kakao Talk had previously indicated to us that operator resistance was one reason why it was holding the launch of ‘Kakao Voice’ in Korea, but it has forged ahead with the move nonetheless.

Mobile carriers worldwide are already suffering the effects of apps like Kakao Talk, Line and WhatsApp which rival them for SMS revenue, and the move to offer voice adds further pressure by giving users a free alternative that is particularly attractive when calling overseas.

The Korean Telecommunication Commission (KTC) is reported to be looking into whether mVoIP services need regulation, and whether they may warrant a change in the country’s telecommunication laws, according to the Seoul Space blog.

Kakao Talk was founded in 2010 and currently serves 1.3 billion messages across its platform each day. Chinese Web giant Tencent led a $80 million round of funding for the Korean mobile app company in April.

Smartphone adoption rates in Korea passed 50 percent in May, with authorities praising the positive effect that apps have had on society and productivity in the country.

Operators in other countries have taken action to combat the effect of apps on their network. Earlier this year, Japanese operator DoCoMo asked Google to modify Android apps to lower the amount of data that they use, in a bid to lower the strain that messaging and talk apps place on its data network.

➤ Kakao Talk | App Store | Google Play | BlackBerry | Samsung Apps (Bada)