Thank you for posting and welcome to the Community. I'm sorry if you haven't been able to set up yourself as an account manager for your Grandparents. Normally the PIN is sent to the email or mobile number linked to the account. It could be that your Grandparents don't have either linked to their BT account.
If you send me your details, I'll be able to give you a hand.
I have sent you a private message with instructions on how to contact the team. You can access your messages via the envelope icon at the top right of the screen, or click on this link, Private messages