Voxer

Voxer is the app for communication on windows phone that allows you to easily and instantly communicate with a friend or group of friends. Friends can listen to your messages during a conversation, or to record and listen to later. You can also send text, photos, and even share your location.
Features:

• Chat with friends with different device Windows Phone, Android and iPhone
• Send voice, text, photos, and messages
• Messages are recorded so you can listen later
• Choose your favorite chat for simple playback
• Chat with one friend or a group
• Create messages even in offline mode
• Receive notifications about new messages
• Works on any cellular data network and wifi - good for international use