AT&T and Comcast today said they have completed a successful cross-network test of a new Caller ID authentication system, and they plan to roll out the technology to consumers later this year.

AT&T and Comcast are among the phone providers implementing the new "SHAKEN" and "STIR" protocols, which use digital certificates to verify that Caller ID numbers aren't being spoofed.

Today's AT&T/Comcast announcement said the carriers completed "an exchange of authenticated calls between two separate providers' voice networks that is believed to be the nation's first." They called the test an "anti-robocalling fraud milestone."

SHAKEN and STIR will work best when all carriers use it, because that would enable Caller ID authentication when a customer of one carrier calls a customer of another carrier. The technology can be used with mobile phone services and many home or business phones, but it isn't compatible with older landline services that rely on TDM instead of IP.

SHAKEN/STIR not a cure-all

Caller ID authentication could ultimately help improve robocall-blocking systems, but the AT&T/Comcast announcement didn't actually promise new blocking capabilities. Caller ID authentication will let phone customers know when a call is coming from a verified number, however.

"While authentication won't solve the problem of unwanted robocalls by itself, it is a key step toward giving customers greater confidence and control over the calls they receive," the companies said. Calls that fail the SHAKEN/STIR test will still ring your phone, but they won't be marked as verified.

The AT&T and Comcast test was conducted with the companies' digital home phone services but not with AT&T's legacy landlines.

"The test used phones on the companies' consumer networks—not in a lab or restricted to special equipment," the companies' announcement said. "It was conducted March 5, between AT&T Phone digital home service and Comcast's Xfinity Voice home phone service."

The test "helps pave the way for AT&T and Comcast to begin offering authentication on calls between networks to customers later this year," the announcement also said. "Customers could soon start to see verified calls not only from callers using the same provider, but more importantly, from other participating providers."

Comcast previously told the FCC that it would roll out SHAKEN/STIR's call-verification capability to its entire residential subscriber base by the end of March 2019. AT&T has promised that capability in Q2 2019. SHAKEN/STIR for Voice-over-LTE calls on AT&T's mobile network is supposed to follow in Q3 2019.

AT&T and Comcast said other carriers will soon conduct their own tests. "Over the coming months, major service providers will be conducting similar tests with each other's systems, verifying that their SHAKEN/STIR implementations are compatible," they said.

Comcast plans expanded blocking

While AT&T and Comcast didn't promise new blocking capabilities today, Comcast said in a November 2018 letter to the FCC that it "plan[s] to begin deployment of a centralized capability for blocking calls appearing to originate from certain invalid and unallocated numbers." This will help Comcast "block a significantly greater volume of fraudulent robocalls" and "move swiftly to add or remove numbers or ranges of numbers to be blocked as the need arises," Comcast said.

Comcast currently partners with Nomorobo to provide robocall blocking. AT&T noted that it offers "free and automatic in-network systems that already label or block billions of unwanted or illegal robocalls." AT&T also offers additional free and fee-based call blocking systems for its home and mobile phone services.

Federal Communications Commission Chairman Ajit Pai has threatened "regulatory intervention" if major phone companies fail to adopt SHAKEN and STIR this year. Most major US phone providers have pledged to do so. The technology would also have to be adopted by carriers in other countries in order to authenticate calls between US-based and international carriers.