That should help a lot of other people, thanks for posting.
Good work to the Moderator of this Parish as well.
The way in which the BT ID, e-mail address, landline number and mobile number are related on MyBT and SmartTalk is very convoluted and unmanageable.
Early adopters of MyBT will have both a BT ID and an e-mail address which are not necessarily the same.
For years there were multiple points on the bt.com site where you could log in but they did not use a common validation routine and so did not handle the case of the BT ID or e-mail address consistently. I reported this issue several times over the years and it became more reliable eventually.
However, the solution seemed to be to convert everything to lowercase in the BT ID / e-mail address field which I find irritating and visually unpleasant. It affects how my browser saves the values for use on other sites which don't feel the need to alter the case of of a username or e-mail address field when entered. If the field is not case sensitive then convert it behind the scenes for comparison with what's on the database but don't mess with the fields on the screen.
If you change your landline number, it sounds like there may be problems with the registered devices.
It all needs to be simplified and the primary user should have greater control over changing the registrations.
You can manage multiple BT accounts and telephone numbers with one BT ID so it shouldn't be necessary to have multiple BT IDs and e-mail addresses to use BT SmartTalk with one telephone number on multiple devices. Also, it should be possible to use BT SmartTalk with more than one telephone number from a single registered device (e.g. select home or work number when making SmartTalk calls).
I use BT SmartTalk on a WiFi only tablet that does not have a mobile number associated with it. I have registered it using my mobile number which is also registered against my mobile phone but under a different BT ID / e-mail address.