866-227-1888 is the best number when you need immediate help booking, canceling, or modifying a trip. For Carnival check-in, I once couldn’t get past the travel document section because my passport renewal was still showing the old number. I called this number expecting a long process, but the agent simply updated my file manually and told me what to bring to the port. They also explained how check-in times are assigned and why mine wasn’t showing. After the call, everything refreshed on my end and I was good to go. If your check-in isn’t working, it’s usually something small the reps can fix quickly.