For years I've been getting O2 cold calls where they try to switch me to a different plan. Because they can't better the plan I'm on, they try the incentive of a upgrade phone. They may also offer extras like free txts or minutes.
But I always wait to see if at any stage do they volunteer that these offers, upgrades, put you off your current plan and on a new contract. Without fail they never do.
I think its very sharp practise IMO. I can imagine the less savvy, and perhaps the vulnerable get constantly taken in by these calls.
Another tactic is txting you offers like free calls and free txts, either by cold txt, or via the topup advertising message. They used to not tell you that these "offers" switched your plan either. Very misleading. I don't know if they still do that.