The biggest cost, in Germany or otherwise, is the people who do the talking to the callees. It dwarfs the costs of RTNR and suchlike.
That has long since (i.e. decades ago) been optimized, both by mechanizing the Hell out of the call filtering process and enabling remote operations where labour costs are cheap; and mass telephone calling is really not a counterexample to the claim that expense makes such things difficult.
Indeed, what has happened, rather, is that a demand for outbound call centres has given the world cheap off-the-shelf set-ups for this stuff, complete with everything from answer 'phone detection to agent activity/performance tracking.
