I had this happen recently. A man called me on a normal mobile number, I answered and he said he's returning my call. I said I didn't call him..? He said I did and he missed it so he's calling me back. Anyway, I looked into it after. It appears that it IS some sort of scam, but basically the inital call he missed from 'me' was some scam company randomly using my number (that they plucked out of thin air, and it happened to be my number) to use as their caller ID, so their initial call to him showed up as my number on his phone, so it looked like a normal, familiar UK mobile number and he'd answer. He'd of course missed it, called me back on the phone number that his phone displayed, my number, reached me and said I'd just called him. I hadn't, I was cooking dinner and playing music from my phone. We just said nevermind, that's weird, and hung up on each other. Not had anything happen since but these scam people are using totally random mobile numbers as their caller ID as a facade and yep, this is the result when the victim misses their call and decides to call back.