I have a customer that has had a DECT V10000 cordless unit in place for approx a year. It is in an auto shop, so the environment/use of the handset has been pretty harsh. They can no longer hear anything from the handset, very faint. Can you purchase a replacement handset rather than purchasing another whole complete V10000 unit? If so, where?

I seriously doubt it. Have you checked with Aztec to see what they can do about repairing it?