@DaveHarbud wrote:
It's around 20 years old
If it is early 2000's or before that is probably your issue.
The telephone master socket on the wall contains a component referred to as a ringing capacitor which used to provide the signal for phone ringing. This might be needed by some older phones. The hub does not have one as it is not required by new phones.
Some have reported on here that the following can be used to test the ringing and confirm the issue. By means of a test only, and at your own risk if you are confident to do so, do you happen to have an old ADSL microfilter to hand anywhere? If so, try connecting it in between the VM adapter to the hub and the phone. Hub > VM adapter > ADSL microfilter > Phone The ADSL adapter should contain a ringing capacitor.
If it works that is your issue. Probably not suitable to leave the filter in place permanently as an ADSL filter is not designed for this type of connection. You can get adapters (RJ11 plug to BT socket) which have a ringing capacitor in. VM may be able to supply one.
Alternatively may just be time for some new phones. As suggested by Paul_DN, try and borrow a newer handset to test with to confirm.