In a report and order, the Federal Communications Commission (FCC) approved new rules that will allow phone companies to block illegal robocalls originating from certain types of numbers.

Building on the efforts of the Robocall Strike Force to address fraud and identity theft that often accompany calls that manipulate or spoof caller ID information, the agency said providers will be permitted to block three types of calls.

“The rules we adopt today outline specific, well-defined circumstances in which voice service providers may block calls that are highly likely to be illegitimate because there is no lawful reason to spoof certain kinds of numbers,” the FCC wrote in the report and order. “Thus, a provider who blocks calls in accordance with these rules will not violate the call completion rules. Conversely, a provider that blocks calls that do not fall within the scope of these rules may be liable for violating the Commission’s call completion rules.”

First, voice service providers will be permitted to block robocalls that appear to be from telephone numbers that do not or cannot make outgoing calls.

The second category of calls that can be blocked without running afoul of the FCC’s call completion rules are those purporting to be from a phone number placed on a “do not originate” (DNO) list by the number’s subscriber. Such DNO calls are “highly likely to be illegal and to violate the Commission’s anti-spoofing rule, with the potential to cause harm, defraud or wrongfully obtain something of value,” the FCC wrote.

Broad support for blocking this category of calls was found among consumer groups, providers, government and callers, the agency added.

Calls that originate from invalid numbers—such as those with area codes that don’t exist, unassigned numbers, and phone numbers that have been allocated but are not currently in use—can also be legally blocked. “Use of an unassigned number provides a strong indication that the calling party is spoofing the caller ID to potentially defraud and harm a voice service subscriber,” the FCC said. “Such calls are therefore highly likely to be illegal.”

The FCC declined to prescribe a sharing mechanism for providers but strongly encouraged them to cooperatively share information.

Additional rulemaking may be necessary, the agency noted in a Further Notice of Proposed Rulemaking, requesting comment on two topics. The FCC asked for input on the optimal method to handle erroneously blocked calls. For example, should the agency create a formal process to challenge a decision to block a number?

The FCC also queried how to measure the effectiveness of its efforts to regulate robocalls, perhaps by instituting reporting requirements for providers (including documentation of any mistakenly blocked numbers).

Comments will be accepted through Jan. 23.

To read the report and order, click here.

Why it matters: Emphasizing the volume of illegal robocalls—one estimate put U.S. consumers on the receiving end of 2.4 billion robocalls per month in 2016—and their spot as the top consumer complaint, the FCC said the new rules are “another important step in combating illegal robocalls.” Commissioner Jessica Rosenworcel filed a statement dissenting in part, pointing out that the new rules failed to prohibit carriers from charging consumers for the call-blocking services.