There's no difference between the standard phone offerings on a trial Vs. paid subscription. If you want to add OpenVoice Integrated however, you will need a credit card on file, as this services does not offer a free trial.
(Canada is one country that comes standard with all GoToMeeting subscriptions).
While the countries codes for GoTo audio services remain relatively static, some area codes within those countries may change based on availability. These areas cannot be predicted when assigned, but will remain static for the length of any scheduled event.
If your country is not currently listed, then I would suggest using VoIP / Mic and Speakers instead of dialing via telephone, in order to avoid any unknown international telecom costs.