Numbers that put out a lot of calls?
And how do they determine that? Probably most spammers are using some VOIP gateways to route their calls through, and probably have the ability to switch between them quickly. Once the spammers figure out how BT's system operates, they'll quickly work out how to get around it, and BT will always remain a couple steps behind.
I don't see how stopping spam in phone calls is all that much easier than stopping it in email, and we all see evidence in our inboxes that email spam is still very much a thing.