delves deeper into voice calling with iPhone support

All-in-one instant messaging service is expanding beyond its text roots into the world of voice. On Monday it announced a new update for its iPhone(s aapl) app, supporting free VoIP calls between imo users on either cellular or Wi-Fi connections.

The iPhone update complements the VoIP calling feature it added to Android(s goog) back in February. There’s no way of dialing out from the service, which only works between VoIP-enabled clients, but doesn’t face the same network effect problems of other early stage over-the-top IP communications startups. Since ex-Googler(s goog) Georges Harik and his brother Ralph Harik founded the company in 2007, it has already built up a healthy customer base for its primary IM service.

Imo bridges the IM protocols of 11 different chat platforms, from Facebook(s fb) and GTalk to AOL(s aol),Yahoo(s yhoo) and Microsofts’(s msft) instant messengers, allowing customers to send text, multimedia and even short voice messages to any of those users through a single client. According to the company its app has been downloaded 4 million times on the iPhone, iPad, Android, BlackBerry(s rimm) and Nokia-Symbian platforms, and it averages 700,000 million daily users sending a combined 50 million messages a day.