happened to me twice, about 6 months apart.
I was worried they were going to try and intercept the phones on the way back to O2 (in my case) so I took the phone to a store and asked them to handle the return for me after gioving me a receipt for it.
It’s very difficult to get info out of the phone company (I couldn;t even get a returns number as I didn’t know the answer to the security questions – ‘But it’s a fraudulent purchase how would I know the answers’ = ‘I’m sorry sir but we can’t discuss the matter with you as you’re not the account holder’).
The police aren’t interested as the phone company are the victim of the crime, not you and they haven’t reported it.
Frutsrating experience all round, really, and never did get an explanation.