Oh feck me mate NU are bast4rds to get any kind of message across to. I did it like this:
Call the number, wait for the person to answer... Say "Can you put me through to your supervisor please?". When they ask why, say "Can you put me through to your supervisor please?". Repeat as many times as necessary. Eventually, you'll get through to a supervisor, then say "I need to speak to someone who's first language is English please". Again, you may have to repeat yourself a few times.
It's not that I don't like dealing with people who don't speak English as their first language, but I believe that to fully understand your predicament you have to converse with someone in the same country as you are. Good luck!