True, but presumably your carrier is still paying for the call. If the transaction were reversed, then the robocaller or their "carrier" (or whatever the equivalent is in robocaller land) would have to pay, and the profits would crumble.
As you can imagine, per-minute call billing is not always settled between tandem players. They can block each other based on company ID alone, and often this requires cooperation from the tandem operator as well. Some of the per-minute rates are incredibly high. The flat-rate long distance phone providers are obscuring the fact that some areas are extremely high-cost to terminate. This system has sprung into high per-minute cost billing in high-cost areas (rural LATAs) so that people operate free dial-up (in the 90s anyways), free phone conference lines, and even "free international calls" all taking advantage of the incredibly high per-minute rate by billing the maximum rate permitted in their LATA (typically not higher than the highest per-minute tariff posted by the incumbent local monopoly).
This highly regulated old-school system seems so antiquated in the face of the Internet, but it survives because the tandem is the one place where all providers can interconnect, and this turns out to be the hardest part of the puzzle to work around. In the age of the internet, we could come up with something better :)
These phone calls are no longer expensive, generally speaking, which is one of the reasons it is now cost effective.