(Reuters) - WhatsApp, the world's biggest mobile messaging service, is to add a voice call service for its 450 million customers, laying down a new challenge to telecom network operators just days after it was bought by Facebook for $19 billion.

Chief Executive Jan Koum said his aim was for WhatsApp users to be able to make calls by the second quarter, just as they can now text messages, in a bid to expand the service's appeal to help it hit a billion users.

"We are driven by the mission that people should be able to stay in touch anywhere and affordably," Koum said at the Mobile World Congress industry fair in Barcelona on Monday.

WhatsApp and competitors like South Korea's KakaoTalk, China's WeChat, and Israel's Viber, have punched a hole in telecom operators' revenue in recent years by offering a free alternative to SMS text messaging.

The news that the most powerful of them was adding voice calls to its service will be seen as worrying for telecom operators globally, which got about $120 billion from text messaging last year, according to market researcher Ovum.

Adding free calls threatens another plank of telecom operators' revenues, albeit one that has been declining in recent years as carriers' tweak tariffs to focus on mobile data instead of calls.

Since the advent of the now Microsoft-owned Skype voice over internet service a decade ago and the rise of internet service giants like Google, telecom bosses have gotten used to facing challengers whose services piggyback on their networks but complain that they are not subject to the same national regulations.

Mats Granryd, the CEO of Swedish mobile operator Tele2, said he was happy to work as a partner with the likes of WhatsApp, because of the additional data traffic they generate, but shared the concerns of other network operators that they must live under often strict national regulatory regimes while internet firms were free to do as they pleased.

European industry executives called again on Monday for regulators to ensure a level playing field.

"They (internet firms) need to be regulated a little bit more and we need to be regulated a little bit less," said Jo Lunder, who heads Russian mobile network operator VimpelCom.

Both Facebook founder Mark Zuckerberg, who is due to speak at the Mobile World Congress at 1700 GMT on Monday, and Koum have cast themselves as partners to telecoms network operators,

not just competitors.

On Monday Koum announced a partnership with E-Plus, the German subsidiary of Dutch group KPN, under which it will launch a WhatsApp-branded mobile service in Germany.

"We are working with carriers in established markets to bring value to end users," he said.

Meanwhile Zuckerberg is expected to have dinner with the heads of several top mobile operators on Monday, including Vodafone's chief executive Vittorio Colao.

Asked for his reaction to the Facebook deal with WhatsApp, Colao said that he did not understand how such an important acquisition among internet players could go unchallenged at a time when European network operators were facing intense scrutiny from their competition regulators.

"These types of deal are a clear indication that the world is changing and the regulations don't fit anymore," said Colao on the sidelines of the conference.

He was commenting just hours before the European Parliament is set to vote on Monday night on a package of proposed telecoms market reforms which among other provisions would restrict the ability of carriers to charge internet companies like Facebook to give them an enhanced service in handling their network traffic.

(Post by:- Satish Devnani)