Vonage unveils mobile app for iPhone, BlackBerry

* App will use cellular minutes when not on Wi-Fi network

* App will initially only work on prepaid per minute basis

NEW YORK, Oct 5 (Reuters) - Vonage Holdings Corp (VG.N) plans to offer discounted international phone calls to iPhone and BlackBerry users, as the Internet telephony company aims to expand beyond the residential market.

Consumers could save more than 50 percent for calls to dozens of countries if they download a free Vonage application to Apple Inc (AAPL.O) iPhone and Research In Motion (RIM.TO) BlackBerry devices starting on Monday afternoon, Vonage Chief Executive Marc Lefar said.

"It's an important first step for us. It moves us from the home platform," Lefar told Reuters. "Over the long term mobile is absolutely a critical place for Vonage to be competing."

The service is available on any iPhone, which is available only to AT&T Inc (T.N) subscribers in the United States, or any BlackBerry regardless of the carrier.

Computer-based Internet telephone services have been around for years, but mobile Web calling has come into the spotlight in recent months as Apple rejected Google Inc's (GOOG.O) Web calling service as an application for iPhone.

Lefar said Vonage was careful to follow all the application requirements set out by Apple and Rim.

Vonage users will even be able to make calls from iPhones using AT&T's cellular network as well as the device's short-range Wi-Fi links, found in places like coffee shops.