Refused to pay back garantie.

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What can i do to a company that refused to pay back my deposit with them as garantie. I paid a deposit with them when signing a contract but when i decided to stop with them they refused to pay back the money deposited,though i respected all terms in using their facilities. The amount of money spent on calls that i have made to them and the register letters that i have sent to them is getting close to the amount of money i paid for garantie. What else can i do?