Vonage already offers its customers very competitive flat-rate unlimited calling. But apparently the company feels that making those calls from phones attached to your Vonage router (or the company’s mobile app, if you’re lucky enough to subscribe to a carrier that supports it) just isn’t good enough. Enter Vonage Extensions, a new feature that will allow Vonage customers to make free calls from non-connected phones.

Extensions work a lot like a calling card from your long distance company, or from one of the many dial-around operators you used to hear about so frequently. Add an additional number (like your cell phone) to your Extensions on the Vonage settings page, create a PIN, and Vonage will give you an access number that you can pre-dial to make outbound calls at no charge. It’s a feature with the potential to save many users a hefty chunk of change on their mobile long distance.

You’re probably thinking that calling codes are so last decade, and you’re right. Not to worry, forward-thinking smartphone owners: Vonage has said that it plans to introduce both Android and iOS apps that will greatly simplify the Vonage Extensions calling experience. Unfortunately, no specific date has been given for their availability — only a murky “in the coming weeks.”

The move puts Vonage in more direct competition with mobile apps like Skype, Fring, and the more nascent Viber. There are also a pair of  existing Vonage apps — Vonage Mobile for Facebook and Vonage Mobile for iOS — so it’ll be interesting to see what unfolds when the new apps arrive next month.

More at Vonage, via All Things D