EBay-owned Skype has released into open beta testing a client for mobile phones that supports phone calls, chat, and other features of the popular VoIP (Voice over Internet Protocol) application.

"This product is a part of our goal to be on as many platforms as possible," said Wilhelm Lundborg, product manager, Skype for Business.

About 50 handsets from Motorola, Nokia, Samsung, and Sony Ericsson are compatible with the service. Other phones might works as well, as long as they support Java, since that's what the client is based on.

Right now, all the features work in the U.K., Brazil, Sweden, Denmark, Finland, Poland, and Estonia. Elsewhere, the data features and incoming Skype calls work, but the software does not allow outbound calls.

Just like existing the existing Skype client, offered by, for example, mobile carrier 3, it uses the mobile network to make calls, like a traditional phone. All calls are routed via a Skype-owned gateway, to keep costs down.

The mobile data network is used to send presence information and instant messages.

What a user pays depends on a number of factors. Calling a Skype contact abroad will be cheaper than making regular calls, because users pay whatever their mobile carrier charges for a local or national call, which will be less expensive than regular international connections.

Skype's gateway setup makes the service unsuitable for users who are roaming internationally, however. Roaming users have to pay charges for a connection back to the Skype gateway, which could add up, the company warns.

The price to call a non-Skype user is split into two parts: first, users pay their carriers, as they would normally, and then they have to pay the SkypeOut fee for the destination being called. So the price of calling non-Skype users, abroad, for example, is cheaper than making regular calls only if the SkypeOut fee turns out to be less expensive than a user's regular international plan. To keep costs down for such users, Skype is offering a variety of SkypeOut packages, announced earlier this week.

Skype treats incoming calls like they are being forwarded to users' mobile phones. So users have to pay the SkypeOut fee for calling mobiles in their country when they answer calls from non-Skype users.

Since the client also uses the data network, users have to pay for that as well. Skype recommends that users sign up for a data plan, with a flat monthly rate, according to Lundborg.

Although the amount of data used is pretty small. A person with 20 contacts who's online for one hour, instant messages for 10 minutes and has a 20 minute call each day will use about 1MB of data per month, according to Skype.