I got another email response from BT
Thank you for coming back to me. I have tried to enter you number and it advises it is not eligible. Unfortunately we do not hold the voucher codes. Please contact BT on 0800 800 150 and they can support further.
So I do that and get put through to Brian. He asks me a lot of the usual security questions and I explain the problem.
Brian: <tippety-tap-tap> Which webpage did you use?
Me: The one on your email -
bt.com/DVequipmentBrian: <tippety-tap-tap> I've just asked on Teams and there aren't any in stock.
Me: According to your 'shop' there are more than 50 in stock and I have one in my basket but no free voucher code.
Brian: Well you can text 61998 with the word 'Adaptor'.
Me: How will they know how to send it to me because BT don't have my mobile number?
Brian: Well, you could text them your address or landline number but there won't be any in stock until the end of November...
Me: Why did BT advertise them then, you must have some idea how many people will need them?
Brian: <tippety-tap-tap> Sorry but I'd suggest you text them at the end of November...
RC