Having been (a very long time ago) one of these call centre gimps, as you so eloquently put it, the way it works (or used to work) is this:
Normal Customer service types are given a matrix with average monthly spend and discount entitlement on it. The higher the spend the bigger the discount. When you ring up, they work out the discount you are entitled to, and then take that off the offline price of the phone.
This is a rigid thing, in that you have no room to negotiate with your customer. Also you have no comission or bonus from this. The customer (quite rightly) will tell you to stuff it as they will have seen the phone they want advertised elsewhere for a lot less. So you put them through to the "disconnections" department. Who dont actually do the disconnecting, they are there specifically to persuade the customer to stay with the company (they are often called "save" as that is what they do). and they have a different matrix with different discounts and much more room to negotiate and strike a deal. More often than not they will offer to price match the deal the customer has seen advertised elsewhere.
Why it is(or was) set up this way I have no idea, but you can imagine the grief it caused the normal customer services staff as it was like showing a red rag to a bull. Mind you this was about 6 years ago, so things may have changed by now.
