This isn't something that we have considered so far (which is not to say that we have ruled it out, though

)
We do have a product under development called VoxRoam that works in a similar manner, but instead of using calling cards, it just uses your existing dial plans.
As part of VoxRoam, Voxalot would provide access numbers around the world that you could dial into, enter a pin, and then dial your number. At this stage, I don't know the public release date of VoxRoam though.
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Craig