if it is a Pay As You Go mobile number that you wish to remove from your account just give customer services a call on 789 / 0345 6000 789 and ask them.
They will be able to remove it for you and from my own experience it usually disappears from your account sometime within the next 12 hours after making the request.
If you want to say thanks > click 'Kudos'.Answered your question? > click 'Mark as Helpful Answer' also.
Thanks for joining the Community. I can definitely help you with this and I am able to remove the number from your online account for you if you wish.
I am going to send you a personal message (purple envelope in the top right) to obtain the security details and the number in question and I will remove it right away.