What should I do to make it clear to a credit that I will only negotiate a settlement in writing?

I sent a counter-offer to a debt settlement letter that was sent by Citicorp Credit Services Inc to PO Box 6027, Sioux Falls SD 57117 Attn: Vice President, Greg Stevens. The company then sent me a letter that thanked me for my inquiry on my account and gave me a number to call for the collections dept. I refuse to negotiate a settlement over the phone because it is there is always a risk that the company could twist the information around. Then, it would become my word against the credit card company's. Therefore, I require all negotiations be done in writing. How do I make sure that my letter gets to the vice president of Citicorp Credit Services or a decision maker rather than just some low level rep? What should I do to make it clear that I won't negotiate over the phone, so the company will not send me another stupid letter with just a phone number to call?