So that's how they have been 'back-feeding' the socket? I don't even have the doubler, plus my wall socket had been cut when the line was upgraded in Feb last year (just before the first lockdown - we should have had Gigabit by now if it wasn't for the pandemic!) but that's besides the point..
Anyway, yes, the most obvious way to test if its your wiring would be to connect your land line phone directly to the back of your Hub through the 'doubler'. On mine it's a standard pair of RJ11 sockets, so if you have an old DSL modem cable anywhere (RJ-11 to RJ-11) this could also work! Though as i don't have one i'm unsure of Virgin's pinouts!
Kind regards,
Lee