GoogleVoice has had problems accessing some toll free numbers, which I assume your calling card uses. However, there are many VOIP providers who are cheaper to use than a calling card, if cost is your issue.
Enabling a Voip provider on the OBI and using it for international calls might cost less than a penny a minute with no per call charges, etc. My Voip provider is .7 cents per minute to the UK (less than a cent a minute) and other locations might be a couple cents per minute.