The warning letters indicate that voice service providers SIPphony and Vultik must "end their apparent support of illegal robocall traffic or face serious consequences," according to an FCC announcement.The FCC says its investigations show that Vultik and SIPphony have allowed illegal robocalls to originate from their networks.Each provider must take immediate action and inform the FCC of the active steps it's taking to mitigate illegal robocalls."We will continue to use every tool we have to go after this fraud and stop the bad actors responsible for these calls in their tracks."If you're done with receiving spammy, scam phone calls, check out our advice on how to put a stop to them."