I don't understand cell phone companies here in the US. There is an oft-quoted business rule that "new customers cost way more than existing customers" but cell phone companies basically cause this: they offer new customers free phones and/or buying out contracts from other carriers if you switch. But if you are already a customer, there are basically no incentives. So I'm toying with the idea of seeing if I can convince a salesperson to give me a phone for half off, to keep me around, which is less cost for them than if I was a new customer.
Not only is it rare for there to be real incentives to stay with a company, but (in America at least) the opposite is typically true: If you stay with a company for a long time they slowly jack up your rates cause they know once they have you for over a year you are less likely to switch.
This is why they are willing to pay a bunch to get new people; once they are within their ecosystem inertia means they are unlikely to switch out cause its a big hassle. This goes double for stuff like long term contracts or tying your phone to the contract; it locks you in means they don't have to worry about you leaving during that time at all.
Calling in to threaten to cancel every ~year is indeed a good way to get a discount or remove BS additional charges they add to your bill.
E: Note that this applies to stuff like car insurance, internet, TV and even your gym membership. Worst case scenario is that they say "Lol, go ahead and cancel" but odds are decent they will give you at least a temporary discount.