Re: "huge computing power"
"And presumably if it's redirecting to voicemail, BT are still getting their coin for connecting the call through, so they still get some revenue anyway."
The "huge computing power" is actually just enough storage to ensure BT can record the calls and claim the spammer revenue.
While it doesn't require "huge computing power" at present, once they start selling the automated spamming services to allow both the call and receiver to be connected to the same system and cut out the inefficient third-party systems, the requirement to automatically provide spammers with numbers that then divert to the spam mailboxes will drive requirements through the roof...