https://www.avforums.com/threads/how-do-i-find-out-my-bt-landline-number.1034449/
Dial 17070 from a phone connected in the relevant house. Click to expand... That is how it is done. However, To the OP: It isn't your number. It is the ...

https://www.bt.com/help/landline/how-do-i-find-my-bt-phone-number-
Simply dial your mobile (or a friend or family member's) from the landline. You should also dial 1470 before the mobile number if you have number withhold.

https://community.ee.co.uk/t5/Broadband-Landline/Lost-landline-number-when-moving-from-BT-to-EE/td-p/1380769
I have lost my home phone number. EE say this is irretrievable but I see moderators have been able to help some customers in similar situations.