Not just BT. Also, those teams will almost certainly be aware of the previous contacts (as their systems will record all that). I suspect that it all depends on how desperate they are to retain customers how far they will go in the offers. So what might look random is probably not (also, different teams might have authority to offer different deals).
This is not just BT. It's many suppliers. For that matter, it's rife in many industries. Insurance is a good one.
Of course the irony is that all these special offers have to get paid for one way or another, and that's largely through the hike to more "loyal" customers. It's often called the "loyalty penalty".
So the upshot is, if people can be bothered with the hassle, there are often deals to be done. If you want a quiet life you end up paying more.