I get numerous calls from different numbers that all list Level 3 Communications as the carrier.  These come from places as far apart as Miami, New York, Maryland and Texas. Typically, One number calls and doesn't leave a message then within 15 minutes or so another call comes in from a different number and location but still on the Level 3 Communications carrier and again no message. The pattern seems to indicate illegal robo-calls looking for live numbers. Since they all originate with this specific carrier the carrier is probably as much to blame as who ever their client is.