My Net Fone Netcomm V100LS $79 @ Harris Technology
The credit is only with 'My Net Fone' though.
You can use something like Skype which is cheaper but with the V100LS you get much better voice quality. I have had many a Skype call when no one can understand a thing you are saying! So cheap isn't always better.
You also qualify for 3.15% cashback when you login here before you buy at Harris.