Have contacted them repeated number of times per e...

Have contacted them repeated number of times per email without reply.
When I get them by phone, they ask me to email again. And then again, no answer. Their solution is to send already paid invoices to Debt Collection, which I need to spend time to resolve, even though I have already paid. It shouldn't have been like this. They could have responded to emails, and then we could have easily solved their mistakes.