AT&T and Comcast are partnering to authenticate calls made between the two networks, letting customers know when there’s a genuine caller — not a spammer — on the phone. It won’t cover every call AT&T and Comcast customers receive, but it’s a step in the right direction to battling the scourge of robocalls.

AT&T says the deal is likely the “nation’s first” to authenticate calls between two providers through the SHAKEN/STIR (which stands for “Secure Handling of Asserted information using toKENs” and “Secure Telephony Identity Revisited”) protocol. The protocol lets consumers know when a call they’re receiving is actually being placed by the displayed number listed on caller ID. AT&T and Comcast said they hope to have the system up and running for customers later this year.

The SHAKEN/STIR protocol works by using digital certificates to verify whether a call is actually coming from where it says it is. It’s unclear how you’ll be able to tell whether the call is spam, but it’s likely that an identifier will show up when you receive a call that’s legitimate.

The system has some limitations for the time being. It can only be used to identify legitimate calls — not to detect spam ones. Robocalls will still come through; they’ll just show up as unverified. For now, since only two companies are involved, phone calls originating with any other provider won’t be validated. Until every other carrier deploys SHAKEN/STIR and links their systems together, it will be impossible to identify every legitimate call.

In February, Federal Communications Commission Chairman Ajit Pai applauded carriers like AT&T, Verizon, and T-Mobile that pledged to take steps toward implementing the SHAKEN/STIR protocol into their networks. T-Mobile began slowly launching the system for calls on its network in January, and Sprint plans to test it out later this year.

“American consumers are sick and tired of unwanted robocalls, this consumer among them,” Pai said. “Caller ID authentication will be a significant step towards ending the scourge of spoofed robocalls. It’s time for carriers to implement robust caller ID authentication.”