Hi Fiona,
You seem to have missed a digit. It's 800 760 333
They now have an online facility in English.
It's www.endesaonline.com
You then see a Union Jack and click on that and you can put in your details from your bill [factura] to register
This is not easy, but if you ensure you use all the same numbers and details as your bill you should be ok.
e.g. use the same [mis] spelling of your name. If it's your NIE number on your bill put that in - not your passport number.
If you manage to get in you can download bills and send messages to "Customer Service" etc etc.
Of course you are not guaranteed a sensible or rational answer, but it's worth a try!
If you need more help let me know and I will try to assist.
Tom