question about UCB

i have an account with CITI - it's about at 5 mo now - i called them once or twice a month from the beginning, but since a portion of the balance was at a promotional very low rate (which won't expire until like Sept or so), they could not do any sort of payment plan with me (nor could i do the monthly payments they wanted). so apparently it's now with UCB - they've left me a couple messages and when i called most recently to CITI, they referred me to UCB. Just received a letter from them basically "unless you dispute the validity of this debt, please make your payment....". Have not spoken to them yet, this is unchartered territory.... have worked out payment plans with all others but one (in process with that), and would love any advise as to how to proceed with this UCB. Will they do a payment plan? from the letter, it just says make your payment to CITI. obviously i can't do that - how do i best proceed????

thanks in advance for any thoughts/input/advice!