I use Skype (
www.skype.com) a lot when I am travelling (on a wi-fi laptop and headphones). If you've got access to a computer, you should be able to use Skype.
Free PC to PC calls and penny a minute to call most landlines around the world (calls to mobile usually more because of the termination rates unless it is a called party pays system like the US).
18866.com is a carrier pre-select operator who are arbitraging between what the network operators charges them on a wholesale basis and what they charge you on a retail basis. Like most arb opportunities, profitability gets squeezed as too many players enter so expect a lot of these kinds of companies to close in the next 2 years. In the meantime just make sure that you don't buy too many prepaid credits in advance.