Echo Optics - No Customer Communication

Received a letter sent to my client today demanding payment for a invoice that I know nothing about, threatening to list me as Default Debtor, nobody has ever tried to call me or email/sms as they say they have, I have gone through my devices and only have correspondence from arranging the installation to be done, furthermore I never signed off their job sheet, my client did. Extremely dissapointing company to deal with...