Over-the-top messaging service WhatsApp has added push-to-talk voice to its offering, as it passes the 300 million monthly active user mark.

The rise of WhatsApp and other OTT messaging apps, such as Viber, LINE and WeChat, has been putting pressure on operator SMS revenue — and the addition of voice could have a similar impact on voice services.

Voice Messages has been launched for all platforms and allows users to add recorded messages to WhatsApp conversations. Once received, group members can play the message within the conversation.

There is no limit to the length of the messages, and a microphone image shows when they have been heard by recipients.

WhatsApp uses mobile data or Wi-Fi connections to send messages, meaning users don’t need to use their SMS allowance when having conversations. The ability to have group conversations is another benefit over SMS.

Jan Koum, WhatsApp’s CEO, told AllThingsDigital that users of the app now send 11 billion messages and receive 20 billion messages each day. The difference in the figures is due to some messages being seen by several people within group conversations.

The total figure was 27 billion messages in June, when WhatsApp reached 250 million users.

Koum added that WhatsApp now has 300 million monthly active users with more than 20 million in Germany, Mexico, India and Spain individually.

The number of users makes WhatsApp comparable to many of the world’s largest mobile operators. China Mobile leads the way with 726 million connections, according to GSMA Intelligence, while operator groups Vodafone, Telefonica and Bharti Airtel have connections in the hundreds of millions.