Just document all calls, save all letters, and get a good grasp of the Fair Debt Collection Practices Act and any state-level equivalent (Rosenthal in CA) -- in CA you get to shoot the violators with a double barrel for federal AND state violations.
Having the major credit card companies increase my APR to 30% in 2009 having never missed a payment on anything in my life, while being backstopped via TARP and having literal 0% Fed/interbank lending rates blew my top off and I decided giving them the finger was my individual right of protest and damn the consequences. Little did I know I was entering an exciting world of learning how to beat the bottom feeders at their own game. Never paid a dime, made about $20k suing for violations, and my credit score was above 700 within 4 years and after everything fell off (7 years) it's like it never happened. But I'll never forget it - was pretty damn exciting to be honest.
It's basically trying to get them to trip up and make a mistake - and it blew my mind how often and unabashedly these collection agencies would! Hell even BofA and Chase violated before they sold the debt off and I got an immediate removal from my report and the debt wiped away / no more collections (I was happy just to make things 'disappear' in these instances..didn't put money in my pocket but at the time is was a huge win, as one can imagine).