The IRS refunded our repayment to them, not sure what to do with the refund?

We owe some back taxes and set up a payment plan with the IRS to repay our taxes. We made the first payment on the date specified by the IRS. About two weeks later they sent us back our payment plus interest in the form of a refund. We have tried to call and can't reach a person but have left a message. Not sure what to do. Any suggestions out there?