Federal Communications Commission Chairman Ajit Pai yesterday criticized Sprint, Charter, CenturyLink, and four other phone providers for not committing to adopt a new system for blocking robocalls.

Pai issued a press release and sent letters to wireless carriers and other phone providers, saying that certain companies "have not yet established concrete plans to protect their customers" using the new "SHAKEN" and "STIR" robocall-blocking protocols.

SHAKEN stands for Signature-based Handling of Asserted Information Using toKENs, while STIR stands for Secure Telephone Identity Revisited. The new industry standard isn't expected to completely eliminate robocalls, but it may make a sizable difference and is expected to be implemented by carriers starting in 2019.

"Under the SHAKEN/STIR framework, calls traveling through interconnected phone networks would be 'signed' as legitimate by originating carriers and validated by other carriers before reaching consumers," Pai's press release explained. "The framework digitally validates the handoff of phone calls passing through the complex web of networks, allowing the phone company of the consumer receiving the call to verify that a call is from the person supposedly making it."

Pai's letters say that seven phone providers apparently do "not yet have concrete plans to implement a robust call authentication framework" and asked those carriers to answer a series of questions by November 19. Those carriers are CenturyLink, Charter, Frontier, Sprint, TDS Telecom, US Cellular, and Vonage.

Pai's letters yesterday came a month after 35 state attorneys general urged him to act more aggressively on robocall blocking.

Pai thanked other phone providers for committing to implement SHAKEN/STIR. The carriers that already committed to adopt SHAKEN/STIR are AT&T, Verizon, T-Mobile, Comcast, Bandwidth.com, Cox, and Google, according to Pai's letters. He also asked those providers for more details on their plans.

Carriers that haven't committed to deploying SHAKEN/STIR received a different letter.

"Despite these industry-wide efforts, I understand from Commission staff that Charter does not yet have concrete plans to implement a robust call authentication framework," Pai wrote in his letter to Charter. "I hope that is no longer the case."

Explain yourself

The letter to Charter was almost identical to those sent to Sprint, CenturyLink, Frontier, TDS Telecom, US Cellular, and Vonage. The letter asked the following questions, requesting detailed responses by November 19:

What is preventing or inhibiting you from signing calls today?

What is your timeframe for signing (i.e., authenticating) calls originating on your network?

What tests have you run on deployment, and what are the results? Please be specific.

What steps have you taken to work with vendors to deploy a robust call authentication framework?

How often is Charter an intermediate provider, and do you intend to transmit signed calls from other providers?

How do you intend to combat and stop originating and terminating illegally spoofed calls on your network?

The Commission has already authorized voice providers to block certain illegally spoofed calls. If the Commission were to move forward with authorizing voice providers to block all unsigned calls or improperly signed calls, how would you ensure the legitimate calls of your customers are completed properly?

We contacted the seven carriers about Pai's letters last night. Sprint told Ars that it has "received the letter and will respond to Chairman Pai directly." US Cellular answered similarly, saying it "will be responding to the chairman's letter."

CenturyLink told Ars that it "understands consumers' concern" about robocalls and "will continue to work with our industry colleagues to identify and address the sources of illegal robocalls." TDS said it is "currently investigating our options for implementing a call authentication framework on a multitude of platforms." Frontier told Ars, "We are reviewing the Chairman’s letter and look forward to continue working collaboratively with the FCC and industry to fight the bad actors placing these bothersome, invasive and deceitful calls."

Vonage said it "take[s] this matter very seriously" and that it is "working closely with our third-party carrier partners and the FCC to respond to the Chairman’s letter directly."

We haven't received a response from Charter yet.

Today, Pai followed up with letters to business voice providers that he said are not participating in "industry efforts to trace scam robocalls that originate on or pass through their networks." Those companies are 382 Communications, Global Voicecom, IP Link Telecom, R Squared Telecom, Talkie Communications, Thinq, TouchTone Communications, and XCast Labs

Sprint complained about cost

In previous filings with the FCC, some carriers noted that SHAKEN/STIR won't entirely solve the robocall problem, and they questioned whether the FCC has provided proper authorization to use SHAKEN/STIR.

Sprint told the FCC in October that "SHAKEN/STIR will be helpful to carriers battling illegal robocalls but is not a complete solution."

Sprint also complained about the cost of implementing the system. "Sprint is also concerned about the costs of implementing the certificate management requirements of SHAKEN and encourages the Commission and industry to explore more cost-effective alternatives to the central repository process originally contemplated in the development of SHAKEN," Sprint wrote.

Sprint and others noted that SHAKEN/STIR needs widespread adoption by carriers to be effective. Sprint wrote that "SHAKEN tells us nothing about the content of a call or whether it is legal. It just authenticates origination of the call path and the Caller ID information of individual calls. Without universal adoption of SHAKEN from originating carrier to completing carrier, call authentication will not be passed to the terminating carrier."

Charter's filing in September said it partners with call-blocking company Nomorobo to offer robocall blocking to customers but suggested it needs further authorization to use SHAKEN/STIR.

"As the industry finalizes and implements the SHAKEN/STIR framework, the Commission should adopt rules that permit providers to block calls that fail authentication in cases where both the calling and terminating voice providers have implemented the SHAKEN/STIR protocols," Charter wrote. "Given the robust testing of the SHAKEN/STIR framework that is currently underway, the Commission can be assured that such calls are 'highly likely to be illegal.'"

Though T-Mobile has already committed to using SHAKEN/STIR, it pointed out potential shortcomings in its filing.

"First, SHAKEN/STIR can only provide a positive affirmation of the source of a given call," T-Mobile told the FCC. "It cannot provide confirmation of the opposite—that is, that a call is definitively 'bad' or fraudulent. This is particularly true where calls are carried by international providers that do not participate in SHAKEN/STIR and send calls to the United States through wholesale partners."

T-Mobile further noted that the majority of fraudulent calls likely originate from outside the US and that "adoption must go beyond domestic carriers to also include international carriers to have a real effect on the onslaught of fraudulent calls." Finally, T-Mobile noted that SHAKEN/STIR requires IP interconnection. "[T]us, legacy networks will not be able to source or transmit verification data for calls," T-Mobile wrote.

Disclosure: The Advance/Newhouse Partnership, which owns 13 percent of Charter, is part of Advance Publications. Advance Publications owns Condé Nast, which owns Ars Technica.