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Was it any of the numbers in this link -> Is-it-Vodafone-calling.
As a rule of thumb I'd suggest not to agree to anything from a cold call until you've had a chance to look through it and other desks.
Even if it's Vodafone calling.
Ask if it's a timed offer and for it to be added to your account notes so you can take it up if you want to.
And to be honest with you nothing is ever free !
Vodafone Sales wouldn't block their number.
I'm wondering if it was perhaps a partner of Vodafone.
That said you followed your gut feeling. Well done.
I just got the exact same call but it wasn't from a withheld number, offering me the exact same deal. The number was 01709 918 609 which incidentally is on the list of vodafone numbers, just checked it. But to be absolutely sure, I am planning on calling Vodafone to confirm if the offer is legitimate.
The deal is for £12 a month for 24 months with 2GB international roaming data.