This might have been answered somewhere in this forum, but why do you suggest that we do NOT sign letters that we send to either CA or anyone else? Just curious..........
CAs do not employ the most ethical people on the planet, and some have been known to lift a signature off a letter, and transpose it on to something else to make it look like you signed something that you didn't.
Counterfeiting documents is only the tip of the iceberg. The law is reasonably specific about what they can/can't do as debt collectors, yet many of them don't hesitate to step way over the line. They get away with it because 99% of people don't know their legal rights.
