Oh I love these things. I always give my number. When they phone I go through the motions until they ask me for my card number.
'You want my card number?'
'Yes'
'Why, I've just transferred some Zimbcoins to you, 7 million Zimbcoins' 'You not received them yet' Where are my Zimbcoins, give them back'
'Zimbcoins?'
'Yes, Zimbcoins, Zimbabwe's new official Crypto currency as endorsed and announced by Mr Mugabe'.
At that point the line usually goes dead or you receive the reply 'f*****g a******e.