MCI Residential Services Will not refund overpayment Dallas, Texas

Closed my MCI account in May of 2012. On 1/8/13 my bank mistakenly sent MCI a payment of $375. There was no outstanding balance on the closed account. I discovered the error on or about January 15, 2013 and immediately called MCI. They acknowledged they had received the payment. I was told the money would be returned to me within 10-14 days. It has not been received. Called today, 2/14/13, and spoke to Cameron, MCI Employee #2051. No action had been taken and the return of my funds has not been processed. No reason given. Again, I was told I should have it in 10-14 days but I do not believe them. I have no idea why MCI became my Long Distance Carrier to begin with. It was a surprise when I began receiving bills from them in 2006. But I accepted it as one of those telecommunications random assignment actions. I feel they are trying to keep my money hoping I will give up eventually. It is $375, well over my average bill of $27 so it is amazing that they made no attempt to return it without my contacting them. It is a lot of money to me. They have not kept their word to return it to me, so it appears I will have to keep after them since they are not to be trusted. Please help.