They have multiple call centres.
The indian one is the absolute worst to deal with and it's the one you'll hit most often.
The Filipino one is only slightly better.
The South African one is very friendly, but as effective as the other 2
Same applies to the UK/Irish ones.
They have scripts they must follow. They're not allowed to deviate from the scripts (indians take this literally).
Someone I know who's Ex TT said they also have requirements on the number of calls they take each hour (incentive to get you off then line asap or get penalised) and the call centres are penalised if they don't meet a certain (impossible) percentage of "first fixed" calls, so the operators are instructed by managers to not pass to second line under any circumstances. (Not that "second line" is any better. You need 3rd line to talk to someone who's actually got any technical qualifications).
All in all, the call centres are an experience best avoided. BT is better but only slightly.
Do you want to stop spam, or do you just want to stop receiving it?