The box outside your house is the omnibox. It is where the VM cable for broadband and phone (phone is currently on a separate cable) arrive at you home. From, and within, the omnibox the external cable is (probably/possibly) joined. A coax cable goes down the side of your home to your lounge for TV/broadband and a phone cable goes around the front of your house to the phone point and (presumably) on from there to the alarm panel.
When your phone is switched to a connection from the back of the VM hub, all of your phone sockets will stop working. The only phone connection point will be the TEL1 socket on the back of your hub. In order to keep your phone socket/alarm working you will need some phone wiring modifications. If your cables are accessible and all run outside it should be simple to do with minimal disruption.
A VM tech would need to fit a telephone socket next to your VM hub. From this new telephone socket a new piece of telephone cable would need to run outside following the path of the existing coax cable for the TV broadband all the way to the omnibox outside.
Within the omnibox, the old incoming phone cable from the street would be disconnected from the phone wiring to your study (to prevent the new phone connection from the hub being fed back into the street). Inside the omnibox, the new piece of phone cable from the new phone socket in the lounge would be joined to the existing phone cable running around to your study.
VM often uses an adapter lead from the phone socket/adapter on the VM hub to connect into the new phone socket next to the VM hub. Example below
https://community.virginmedia.com/t5/Forum-Archive/HUB3-to-Master-socket-adaptor/m-p/4507007#M262932...
This 'backfeeds' the new phone line from the hub into the new bit of telephone cable to the omnibox and from there it is linked into your existing phone wiring so re-enabling the phone sockets and alarm from the new phone socket on the VM hub.
There are other possible ways to make the connection but the above is likely from what you have described.
As far as you alarm is concerned, it has been noted on here many times over that alarm companies have not been shy of trying to sell new alarm systems off the back of the phone switchover.
If your existing alarm contains a simple voice-dialler which dials out on the phone line to your mobile and plays you a recorded message of an activation then this is likely to work as before. A past topic noted some issues when the alarm was sending digital data to a SMS text messaging service for notifications.
A potential issue with the new arrangement though is that the phone line from the hub is generally dependent of the operation of the VM hub. If there is an issue with the hub then your phone line may also no longer work and your alarm may not be able to dial out.
You would have to agree any/all of the above with any VM tech who visited. There have been very wide-ranging reports of the abilities of VM tech's sent to do this work. Some have received glowing praise for a job well done. Other reports describe the person sent as being totally clueless about how to do the work and multiple visits being required before the task was completed by someone competent.
Having an idea of what is involved will certainly help you if you get a less-capable tech turn up to do the task.
Any switchover might be put off for the time being as VM seems to have suspended the programme after some past failures with vulnerable customers
https://community.virginmedia.com/t5/Home-Phone/Phone-adapter-delivery/m-p/5459483#M202107
