Sorry to hear you lost something you wished to keep.
I'm afraid I don't know any method & strongly suspect it's gone forever. So far as I'm aware, voicemail is stored on Virgin's servers rather than your phone, once you tell them delete it they do so, & unlike your own device where there'd be some hope of undeleting it, the space is likely overwritten almost straight away.
If you were the CEO of Virgin, MI5, MI6, or the PM, or similarly important, you might persuade them to find the recording in a backup, as a normal mortal, I suspect that's a lost cause.
Something I personally wish voicemail was (gone forever that is), that's a different issue I wasted too much time trying to arrange without success... You'd think turning voicemail off would be something you could do in your account settings, apparently you have to do it after ages waiting & talking to "support" operatives who can neither understand English, nor speak it clearly, meaning you effectively can't.
