The EBUL is an emergency backup line but not relevant to your home alarm situation (see pages 35 to 37 in the home phone guide below).
https://store.virginmedia.com/content/dam/eSales/Downloads/Phone%20User%20Guide.pdf
The issue of existing phone extensions not being reconnected during a VM phone installation via the VM hub is a regular source of complaint on the forum. It seems to be a very hit and miss process as to whether it is done or not and it is unclear what the official VM line is. Suspect it may well be down to the individual technician who turns up and if they happen to have the relevant skills/equipment/materials/time to do it on the day. It's an area of the VM installation process which seems to lack any consistency in what can be expected.
If your legacy telephone socket and the VM hub are in the same room as each other, one would have thought that would have been a situation where they should have been able to offer to do that for you as part of the installation.