I have had exactly the same problem as the original poster. After encountering the same error message for days I eventually accepted I would need to call the helpdesk number. After waiting an hour on hold I spoke to a nice lady who said a) she would send activation details directly to the email address I provided (she has not, this has not arrived) and b) the error message on the BT website would in any case be fixed later today (it has not).
I would greatly appreciate someone providing a response or suggested next steps that does not involve me calling a helpdesk number and waiting 1 hour + in a queue (something I have already done with no successful output). Is there not someone I can simply email my details to and they can sort this out? This feels like an extremely straightforward thing to fix, to provide me with a service I have already paid for. Your assistance would be very much appreciated - many thanks in advance
