I have some recent experience of this. I booked a last minute Avios redemption a few weeks ago using a companion voucher to watch my football team - Newcastle United - play a pre-season friendly in Philadelphia. I enjoyed the match and experience so much I decided to stay on for the next game in Atlanta, so called BA around 21:00 ET (just over 24 hours before departure of my scheduled flight back to the UK) to change my ticket. However, the 247-9297 number I called returned a "this number is not in service" message, and no other US number I could find would work either. The UK Silver line number was closed so I eventually ended up calling 0207 949 3064 and, after some pushback, the agent there was able to put me through to someone else who could change an award ticket. Not the easiest experience! I would have thought the US number would have been open at 21:00 ET or at least played a recorded message saying they were closed so is the number perhaps blocked to callers from the UK, which it assumed I was despite roaming in the US?

Of course when I turned up for my rebooked return journey the ticket number hadn't been properly matched to the new flight details but that's a whole other story