HipVoice is a service that allows for Push-To-Talk between Windows Mobile, Windows Smartphone, Windows Desktop, LiMo and BlackBerry devices. HipVoice is also finishing up work on Android, Brew and the iPhone.
Because HipVoice is cellular network independent you can have Push-To-Talk service between different platforms and carriers. That means AT&T customers can Push-To-Talk to Verizon customers.
HipVoice allows for group Push-To-Talk conversations and stores the conversations on your device in case you want to listen to the conversation again.
Sign up for the service at the link below.