That was my question! The issue is that the robocaller is hanging up early, because they get billed on 6 second intervals. But they pay so much more for their bad traffic that it's profitable to absorb a 6 second interval on the intermediaries end, just to avoid the short-call penalties. Yes, if they'd hold the call open themselves, instead of the middle-man, they'd get lower prices overall.

I see things like this in telecom too much. Where people are mathematically going to lose, guaranteed, but prefer a certain billing because it's easier to understand, or looks cheaper. While this is understandable for an SMB that isn't in the telecom business, I've seen big names take these same deals. And I've seen the other end: they 100% are losing a few % in doing so.