Iowa Attorney General Tom Miller on Wednesday called on five major phone companies to offer call-blocking technology to customers who want to block intrusive robocalls. In a letter that Miller and 44 other state attorneys general sent to the chief executives of AT&T, Sprint, Verizon, T-Mobile and CenturyLink, the attorneys general said a new Federal Communications Commission rule clarification allows telecommunication service providers to offer customers the ability to block unwanted calls. The FCC action also verifies that federal law does not prohibit the services. Phone carriers have previously claimed that FCC rules addressing call-blocking technology were either ambiguous or even prohibited it.