Looking at Voipcheap.co.uk it looks like a UK company, though it is, it can be used basically anywhere you get a an internet connection.

Where internationally can you call for free with Voipcheap.co.uk?
Australia, Belgium, Chile, Cyprus, Denmark, Estonia, Germany, Hong Kong, Italy, Japan, Malaysia, New Zealand, Norway, Panama, Puerto Rico (+mobile), Russian Federation, South Korea, Spain, Sweden, Switzerland, Thailand, and United States (+mobile).

Completely free?
Almost, you are limited to 5 minute “trial calls”. Buy any amount of credit and you will get unlimited access to free international calls to those locations.