Faulty landline phones

I applied for landlines for my offices at MFP and every week there was an issue , the phones were not working. I was getting countless messages from clients complaining that they can’t get through to my office to make bookings. I would call MFP and send emails, someone would come but the phones were still having the same issue. When I emailed them wanting to cancel they didn’t respond instead they would come to my office and make promises. I sent a cancellation notice and they are telling me I still need to pay rental of the phones . I don’t understand why I’m told to pay for rental of the phones as the I wasn’t getting service that I was paying for. I was losing clients because the phones were forever faulty. They failed to provide what I was paying for and should therefore release me from their contract!