12voip why cant pay thru paypal and visa prepaid card?

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i got an account of 12voip.and i want to buy some credits,at 1st they say i can pay through paypal,then they block paypal,so i did visa prepaid card,to use for visa-pay-method..but then it still doesnt work..they say some problems with my bank,the problem is that,i can use my card to purchase anything online accept 12voip..pls help me..thanks :-(