Hi JJ, When you ring BT you must insist that you signed up to their 'All Fibre' package on the condition that your old telephone number would be PORTED across!
PORTING is a technical term used in telecommunication jargon. It is the process by where the software in the telephone exchange can enable/disable your telephone number being transferred. This could be to a new ISP (Internet Service Provider), Virgin Media or Vodafone for example.
It might just be in a couple of years time one of the other ISPs is offering a better Broadband deal. To reiterate, if you do swap to another company, then insist upon your old telephone number being PORTED across. Hope this Helps.