Whistle brings free calling to other Whistle users to your Mac.
Whistle provides low domestic and international rates to and from your Whistle number on landlines and mobile phones.
We offer 20 minutes of free calling to and from numbers in the contiguous 48 US states as a chance to try Premium Whistle.
Features:
✓ Make and take calls from your desktop or mark yourself as unavailable
✓ Choose your look and feel from seven different skins
✓ Has built-in integration with your Mac's Address Book

✓ Secure, encrypted calling prevents snooping on public wireless networks
✓ High definition calls to other Whistle clients sound spectacular
✓ Includes two-line calling features, including call waiting and three way calling
✓ Forwards your Whistle calls to any phone number via our call forwarding service for a low per-minute rate
✓ Support for In App purchases through the Mac App Store
And, one note: Your best experience is going to be with headphones.