If VM have any outbound retentions agents working from home or using their own mobiles, it's entirely possible this is genuine.
Personally, I'd like to think outbound retentions use "presentation number" caller-ID so as to call "from" a recognised VM contact number. It's not an infallible system, but it would go a long way to reassure customers and deter casual scammers.
Whether they do or not, I don't know.
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