Battling in the "We're not Skype, we're better" over-the-top (OTT) services category, Rebtel (News - Alert) and Viber are both announcing enhanced clients. Rebtel now has HD voice support, while Viber has added three new platforms, plus updates to its Windows Phone and iOS clients. Expect more software upgrades from the OTTs to come as talk of Rich Communications Services (RCS) builds.

Rebtel has added Google's open source iSAC codec into its latest suite of Apple (News - Alert) iOS and PC applications, a codec created by Global IP Solutions (GIPS). Google bought GIPS in May 2010 and put iSAC into the open source WebRTC project in June 2011. 

The iSAC codec supports sample frequencies up to 32 kHz, according to the WebRTC project, with an adaptive and variable bit rate between 10 Kbps to 52 kbps.   At least 235 customers licensed iSAC for their VoIP/voice clients because of its ability to adapt to limited bandwidth conditions. Licensees back in the day included AOL, Baidu, Citrix Online, Communigate, GoogleTalk, IBM Lotus Sametime, Nimbuzz (News - Alert), Mail.Ru, PCCW (Hong Kong), WebEx, and Yahoo. More recently, iSAC appeared to support HD voice in the Vonage Mobile soft client released in February 2012.

Like the rest of the VoIP OTT free client world, HD voice calls between clients are free. Rebtel says it has over 17 million callers and is on a run rate of over one billion minutes per year

Not to be outdone, Viber says it has passed over 100 million users across the many phone platforms it supports. In the statistics race, Viber claims it handles over two billion minutes of calls and over six billion messages every month, as well as over 10 million calls per day.

New platforms added in Viber's announcement are the Samsung Bada and Nokia's (News - Alert) S40 and Symbian platforms.   Viber says the new platforms encompass a "huge and untapped market" for Viber with over 1.5 billion devices Nokia S40 devices sold worldwide as of January , but I'm a little skeptical how long the Symbian and S40 will remain viable with the push of Android (News - Alert) softphones and Nokia's loving embrace of Windows Phone.

Speaking of Windows Phone 7, the newly released Viber Windows Phone 7 on Nokia devices supports free HD voice quality phone calls and group messaging in addition to free text and phone messages. Viber developed the app "in partnership" with Nokia, according to the company's press release, and the app is currently available exclusively on Nokia Lumina phones.

Viber's update to the iOS platform is a bit more mundane. Viber for iPhone now supports localization to Arabic, Traditional Chinese, Simplified Chinese, French, German, Spanish, Russian, Hebrew and Portuguese.

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