Hypothetical scenario (although I will be trying it out).
Wife's got an android mobile with Tesco sim and has the Xtras app on it (that shows adverts whenever you unlock the screen) and saves £3/month off the bill.
Now there's no such app available for iPhone but she now wants to use my old iPhone with her Tesco Sim.
Now the android mobile will be without a sim but still connected to the wifi as a spare fone. The Xtras app is still working as normal (showing ads).
Is it able to establish that there is no sim in there? Can I still get the £3 discount?
In theory we are still viewing the advert at least 21 days a month (just by unlocking the screen once a day)