Hi
If you receive a call or message from 08456 021 111, this is the BT Text service letting you know you have a message waiting. Call 08456 021 111 to get your message.
For more information on this please see https://www.bt.com/help/landline/set-up-and-use-bt-text
Also please check out the link for other numbers that BT could contact you from - https://www.bt.com/help/landline/i-ve-just-got-a-text-from-61998-or-60204--is-this-a-genuine-mess#:~....
Thank you