At one point in the past, there were quite a few topics about this on the forum (sadly now gone into the forum dustbin/archive) so there are no useful topics to refer back to.
The Beocom phones are now 'vintage' and, based on what I can recall from the past discussions, I think they were designed specifically for the line conditions and signalling that would be found on a traditional BT exchange line from that time period. I assume that the electronic device mentioned above takes the incoming phone signal from the VM hub and converts it to a format that the Beocom phones can recognise.
I would suggest that you should make contact with the firm mentioned to investigate prices for the device mentioned if you want to continue using the Beocom phones.
Alternatively you could look at a modern cordless system to use with the connection from the VM hub.
In the meantime (if you need something to use on a temorary basis) you could pick up a cheap corded handset until you can sort out some sort of cordless replacement or a fix for the Beocoms. You can get a cheap corded phone for around £10 or maybe a friend, family member or neighbour can lend you a phone while you work out a permanent resolution.
VM has just been fined £23.8 million by OFCOM for failing to ensure the switchover was carried out appropriately for vulnerable users.
Do you have access to mobile phones? If not, VM should offer to provide you with a suitable (corded) backup phone which also has the ability to make calls in a power cut.
A VM person should reply to our topic (usually within a few days, or sooner if someone flags it). They won't be able to get your Beocom phones working but they should not have left you in a position with no working cordless phones and no explanation of other possible options.