I read this on Martin Lewis and was a little dubious but can report that it works (not sure on iPhone but definitely on android)
Order a free O2 sim, put it in your unlocked phone and download the O2 Priority moments app. Then take out the sim and put yours back.
I did this 6 months ago and have been getting loads of freebies including a cafe Nero hot drink every Tuesday. I also bought a reusable cup so I get two stamps so every 5th week I get another free drink despite spending no money! I have had £5 mains at pizza express and freebies/cheap stuff at WH Smith.
The only thing I can’t do is enter the competitions. It’s great though.