If your telephone connection is via the back of the VM hub (21CV connection), and you want to add a second phone for an upstairs extension, then your best bet would probably be to look at getting a cordless phone system.
You would plug the cordless base station into the back of the hub and then have the cordless extension (wire free) upstairs where required. You would just need a mains power socket nearby to keep the upstairs phone charged up.
You can buy cordless packs with different numbers of phone extensions and, in some cases, can add more cordless extensions to an existing system if required.
Alternatively, it is possible to hardwire extension sockets from the back of the hub but can be a bit involved for a DIY job. VM could also probably add telephone extension sockets for you but there may be a charge for that which is typically £99 for cabling modifications.
A cordless system is probably your best bet to simply add an extension to an existing 21CV telephone connection.