Sorry to hear that. I never had to call customer service for any of my credit cards for last 20 years (knock on wood), so I can't say if CITI is better or worse. I hope things will get resolved soon.
I would like to share my experience just in case…
I had my existing CITI card account. After registering my Costco card, My old CITI card blocked me out (and asked to register again). I had to go through registration process once again. Once I logged in, I added Costco card to my existing CITI web account. Now I manage 2 CITI cards from 1 web access. All done from website user interface.