I did use the CX whatsapp number at the weekend. On a round the world itinerary about to start, when I tried to check in for the first flight I got a message saying that I couldn't check in to JFK but giving no reason for it - given that it was the US I thought I'd find out the reason in case it was important.
Slow process, it variously told me that they were very busy and to expect a long wait, then that I was a diamond passenger and they would prioritise me, then that it was a 2 minute hold....which went on for an hour until the system kicked me out. I did get through once early on to send a message and they replied three hours later (while I was making tea), got no reply and kicked me out. Much later in the day I tried again and did get through to a live HKT person so I asked what the reason was that I was unable to check in for my NY flight. I was told that because I had a bookable upgrade on the last flight of my itinerary I couldn't check in for any flight in my itinerary. I wasn't about to argue, happy that it was nothing important like an ESTA or Global Entry. But that explanation seems odd, I used to do ex-TPE flights very often and use an upgrade voucher on the long flights reasonably often but never had any problem checking in online, printing boarding passes etc so I wondered if anyone else had encountered a similar problem and been given the same answer?