lT-Mobile and Sprint can now verify that calls coming from one another’s network are genuine and show customers that an incoming call is real. The process is meant to protect against spam and robocalls, and it should help where the feature is supported.

The verification program, using a system known as SHAKEN/STIR, validates that calls from another network came from an actual caller on that network. That allows the receiving party to see some kind of symbol or alert informing them that the call is verified as real, theoretically encouraging them to pick up the phone because they can be relatively confident that an actual person is on the line.

Every phone service provider needs to set up the system and integrate it with every other service provider in order for it to truly work, though. That means that a lot more needs to be done before the system is fully up and running, and it won’t truly be beneficial until then.

Wider support should be coming in the next couple of years. Last month, President Trump signed a law that requires phone service providers to implement the verification protocol, though they have a year to get started. The law comes after years of the Federal Communications Commission pressuring carriers to do something about robocalls but largely trying to make it happen voluntarily.

T-Mobile has already started verifying calls between its customers as well as with AT&T and Comcast customers. But it still doesn’t validate calls from Verizon customers, and AT&T, Verizon, and Sprint have announced even fewer call verification partnerships with major providers so far. Sprint also announced today that it would authenticate calls with Comcast.

Until that happens, the benefits are limited. T-Mobile will show a notice saying “caller verified” on phones when it can verify a call. But many incoming calls still can’t be verified, which means you’ll still have to answer the phone if you want to find out if it’s spam. (Sprint doesn’t say how it’ll show that calls are verified.)

There’s one other complication: not all phones support showing whether a call is verified or not. T-Mobile says it now works on 23 phones, generally more recent high-end devices like the Galaxy S10.

T-Mobile and Sprint are also in the middle of a merger that, if approved, would make this verification process a little less meaningful since they will ultimately be sharing one network anyway. The merger still hinges on the results of a lawsuit brought by a number of states seeking to block it over concerns it would decrease competition.

Ultimately, SHAKEN/STIR is an important protection for phone providers to start building in, and its ubiquitous presence can’t come soon enough. But until nearly every genuine call is going through a verification process, there’s still no telling whether customers should answer the phone.

Update February 4th, 2:05PM ET: Sprint and Comcast also announced plans today to authenticate calls across their networks.