Vonage has released a new app for mobile devices as part of its ongoing rivalry with VoIP giant Skype. The app, Vonage Mobile, is available today for iOS and Android devices. Vonage Mobile offers free app-to-app calling, and claims to offer significantly lower rates on other calls than either Skype or mobile carriers on international calls.

When you launch the app you’re prompted to enter your mobile phone number and email address. The first so Vonage can use your mobile number for outgoing calls, and the second for account management purposes. Also, Vonage uploads your phone’s address book to their servers. Unlike some other apps I could mention, Vonage Mobile is kind enough to warn you that it’s doing so.

Vonage Mobile allows users to make app-to-app calls over 3G, 4G, and Wi-Fi. Calls to other Vonage Mobile app users are free, as are calls to any Vonage number. Interestingly, the app does not require you to be a Vonage customer to make calls. Credit for international calls can be added via in-app purchase.

Vonage Mobile is a free download from the Android App Market and the iOS App Store. The iOS app is compatible with the iPhone, iPod Touch, and iPad. It is not, however, a universal app, which means it isn’t optimized for iPad, only compatible. Check it out and let us know what you think in the comments.