I also had to deal with them. They have been a fighter, but I have their DCA's right where I want them. I sent mine to:
Vikram Pandit - CEO
Citibank (South Dakota), N.A.
701 East 60th Street North
Sioux Falls, South Dakota 57104
CC: Collections or Agent who mailed letter (either one)
Just make sure you always address it to the same person, no matter what they say in the return letters. Remember, if they don't give you what you ask for, ask again. They will try to scare you, FEAR NO ONE!
Vikram won't contact you, but someone on his behalf will. Keep sending to Vikram and just add all other names in CC: area. This is how you will get the tacit agreement in place.
Since I did my 3 letters and estoppel, it has transferred to 4 different DCA's, and each time I have done the same thing. They have even tried the lawyer route which I found out was another DCA. What a joke:)They are getting pretty poor at playing this game. Good luck!