Pay in full; and always alert credit companies of address change

Pay in full; and always alert credit companies of address changes. In this case, if you did and the credit card company still sent to your old address, they are at fault. However, they are correct that your account is sold (the bad debt has been way too long) so you need to talk to the debt collector. That's fine if you had all the time in the world. But why waste time on this for less than $30?