WIFI Calling - Walkie Talkie is the best freecalling service, WIFI Calling - Walkie Talkie is a Push-to-Talk appwhich allows you to talk for free over wireless networks(lan).
With this application, you can transmit your voice data to alldevices which are on the same network or to any device which youknow its IP address.

How to use :

If you want to send voice message to a single user, press thewalkie talkie button and set IP address that you want to talkwith.
Push to speak button, hold and speak.