Skype's Swedish rival in providing VoIP services, Rebtel, has announced new beta software which will allow developers for Android and iOS platforms to bring voice calling functionality to smartphone apps.

The new Rebtel SDK allow developers to bring app-to-app voice calling over 3G and Wi-Fi networks in just 15 minutes, the company says.

"The Rebtel Voice Platform and its powerful Software Development Kit is a library that makes adding a social voice layer to your Android or iOS app as easy as can be," said the company on its Web site. "The SDK has been designed from the ground up to facilitate simple and quick integration by cleverly wrapping all the signaling and codec handling into one entity. This approach lets you focus on your app's user experience rather than the nitty gritty technical details."

The new functionality has the potential to add synchronous communication for gaming or social apps so users can talk app-to-app while they play games or share photos together, without having to sign up on Rebtel.

The new platform means apps can become more engaging and developers can even create their own branded mobile VoIP apps without building a separate back-end.

Rebtel is offering the service for free for now, though it could look at charging later for apps that do a certain amount of volume.

The company is currently the world's second biggest VoIP application in terms of members, boasting 17 million users - still a long way behind the number one VoIP software, Skype, which boasts over 600 million users.

Microsoft bought Skype for $8.5 billion in 2011, and has since turned it into the central messaging platform for its Windows 8 and Windows Phone 8 operating systems.

Rebtel CEO Andreas Bernstrom told Reuters that at least 10 percent of the 1.2 million apps in use would benefit from the new app-to-app call feature.

"If we can get a tiny, tiny percentage to actually start using it, then there's going to be a lot of traffic going through our network," Bernstrom said.

Rebtel is testing the new platform with just three companies, including VIVfone, which makes a mobile CRM app, and MobisleApps, which develops lifestyle, dating and productivity applications.

The company says it hopes to generate around $80 million in revenue by the end of this year.