With Skype for Windows Phone you can interface with over 200 million people that use Skype each month on multiple platforms. Skype for Windows Phone can be used across 4G, 3G and Wi-Fi networks and supports 18 languages.
Skype for Windows Phone works with Windows Phone 7.5 and higher (Mango). Unfortunately you still can't receive Skype calls while the app isn't running on the phone. The new version of Skype includes a new feature that lets you search your contacts and add them on Skype. Included is the ability to dial landline phone numbers, quicker boot times and other fixes."