Some estimate that spam calls make up over a quarter of those received by consumers, creating a problem so virulent that many consumers refuse to answer calls from unrecognized numbers. Despite a sub-industry arising offering robocall-blocking apps, spammers persist, knowing they only need to be successful with a tiny fraction of call recipients. According to Truecaller, one such spam blocker company, phone scams calls cost Americans $10.5 billion a year.