I have tried to use my prepaid sprint phone cards at our cabin on our land line phone. We don't have cell service there. It only gives me a busy signal when I dial the 1-800-814-3718 number on back of the card. I also brought the cards home and tried from my landline phone at home and same I get a fast busy signal. The customer service number on back of card is disconnected. The cards state they don't expire. Where could I call to talk to customer service? Help,please!