If someone new has the phone number now and they have registered an iCloud to that number, Apple should be able to see there are two accounts on the number.
If you are able to tell them that one of them is no longer you, that helps them know who they need to call.
If someone from Apple called me up and said “Hi, it’s Apple here, are you Nath from somestreet in Perth?” I’d be a lot more cooperative. I wouldn’t volunteer anything, but if they knew that I was the account holder and knew I registered it after you no longer used it, and then asked me whether I had another iCloud account, I’d freely volunteer that I did not.