AT&T and T-Mobile have started rolling out call authentication technology on calls placed between the two networks. Soon, if a phone call is made from one network to the other using a compatible device, then the recipient will see a “Caller Verified” message, and they’ll know for certain that the call isn’t from a spoofed robocall or spammer. AT&T also says it’s testing integrating the data used by this system its Call Protect service, which it started offering for free to its customers last month.

The rollout is the latest step in US mobile carriers’ fight to stop robocalls, and it comes after Federal Communications Commission chairman Ajit Pai ordered them to implement a call authentication system by the end of 2019. Last month, the FCC criticized what it called the slow progress being made by carriers toward this goal after it voted to allow carriers to block robocalls by default back in June.

A growing list of networks support the call authentication tech

T-Mobile and AT&T’s call authentication partnership is based on the SHAKEN/STIR standard, which can verify whether a caller ID is accurate. T-Mobile was first to implement the standard back in January for calls made within its network, and in March, AT&T and Comcast announced that they were partnering to authenticate calls between their two networks. This was followed by a similar announcement from T-Mobile and Comcast in April. Sprint has said that it plans to test the technology on its network in the second half of this year.

The SHAKEN/STIR standard has its limitations — namely that it can only tell when a call is definitely legitimate, rather than knowing when it’s definitely spam — but as more devices and network providers implement the standard, the ratio of authenticated to unauthenticated calls will increase, increasing the likelihood that an unauthenticated call is spam and can be safely ignored.