Finally we did the trick taking the risk.
We got the SMS confirmation code giving another cell number from here (Chile -but not the mine one that is already associated to my own VoipDiscount account)
So seems that, been my sister in Moose Jaw, Canada, and the mobile getting the verification code in Santiago, Chile (but not associated the number to any VoipDiscount account) doesn't matter to the purpose of pass the payment process.
Any way, I was surprised been my sister asked by a mobile number to verify her identity
now, despite when I bought my call credit I wasn't. I understand company's care about security and fraud, but sometimes they are going too far asking for more and more requirements.