I wonder how many people have a voice ID of:
"Oh, feck off with your nonsense biometric bull-shizzle"
Personally, I tend to just keep silent at things like that and see what the computer makes of it. Voice ID is no more secure than anyone being able to hear any such conversation, ever, which is basically "not secure".
But to be honest, why have 5.1 million people had to call HMRC should be the real question.