A full list of supported countries would be nice.
http://www.twilio.com/pricing/international-calling-rates
Or view all Countries, with exchange numbers, here:
http://www.twilio.com/resources/rates/international-rates.cs...
I was curious so I scraped the data: http://pastebin.com/DAhjhY0y
Most unsupported countries/territories seem to be small islands. The most expensive maximum price is 14.1c/msg in Bulgaria and Vanuatu. The most expensive minimum price is 14.1c/msg in Vanuatu, then 8.3c/msg in Mexico (also its maximum).
They say 150 countries and over 1000 carriers