picture-2.pngFoonz, a provider of phone conferencing services, is announcing a service that lets you easily set up conference calls across mobile phones.

All by SMS.

Free or very cheap conference calling services already exist, including popular FreeConferenceCall, and Skype. Foonz itself already offers a web-based way to set up a free conference number and invite either land-line or mobile users through email, calls or text — like many competitors.

Foonz’s new mobile service is different because it lets a group of mobile phone users make a conference call without forcing the call’s organizer to pre-register and set up the conference on a web site or by phone.

You initiate a call using SMS text messaging. You send a text message from your mobile phone to Foonz. Then you receive a text from Foonz asking you to text back with the phone numbers of the people you want to call. Then your invitees each receive an invitation text that includes the conference number for them to call.

Then everyone calls that number and voila, you’ve got a free mobile-phone conference.

It costs nothing more than the minutes you’re already paying for on your mobile plan. It’s only available for mobile users in the United States, but works with any phone on any carrier’s mobile network.

The tradeoff: This SMS-based invite process takes longer than Foonz’s web-based method for first-time users. You’re forced to manually enter everyone’s phone number within your text message on your phone, instead of on your computer.

Once you’ve used the service at least once, you’ll get a unique number you can call to start a group call, or send a group voice message.

Users already include small businesses, family calls, clubs and teams, and distributed prayer groups, the company says. It now has 20,000 active users — though, considering that the company has been around since last year, this is quite low.

Other features include a way to call in and leave a message for other members when you’re not able to participate. If you’re the call organizer, you can leave a message for missing invitees: When the call ends you can do so when the call ends — then they’ll receive the message when they call in to the group.

Foonz is a service provided by Maynard, Mass.-based RPM Communications.