Harrased for account that is with another Debt Collector

I have received a number of calls from Nimble and informed all the consultants that the MTN account they are trying to collect for is already with MBD and has been with MBD for 2 years. This means I have been paying the account for 2 years with MBD and thus the balance with Nimble is the balance with MBD as a result of the payments and not the initial amount.I was asked to send a confirmation letter from MBD which I have sent to *** but no one seems to know what I'm talking about as the consultants are clueless and can't provide answers. MBD has confirmed that the account it still with them, hence they send statements and the confirmation letter.I have received no feedback or acknowledgement of receipt from the customer service department. I sent what I was requested to send yet the harassment persists.