I've watched two family members try to sign up for mobile accounts this afternoon - different details/cards/devices yet both fail at the payment stage - is there some issues with the system for sign-ups?
We're currently not aware of any issues with our website however, I would advise to try the following:
Delete cache and cookies
Try a different web browser
Ensure that the card details match that of the account holders address
Just to advise, if your trying to sign up through a 3rd party website that re-directs you back to www.plus.net, you would need to contact the website themselves as this could be an issue with their internal systems.
Thanks for the information though and I'll certainly feed this back to the relevant department!