T-Mobile is trying to help its subscribers dodge more spammy calls.

The carrier is going to begin warning subscribers when an incoming phone call appears to be from a scammer. If a scam call is detected, the caller ID will display as “Scam Likely,” giving subscribers a heads up before they answer or the chance to just ignore it outright.

T-Mobile will also let subscribers block all suspected scam calls so those calls never reach their phones in the first place. But subscribers will have to actively opt in to the blocking service, as there’s a chance the carrier could accidentally filter out legitimate numbers.

Scam filtering is something the Federal Communications Commission has been pushing for phone companies to pick up in recent years. During the Obama administration, the FCC passed a rule change that cleared the way for companies to start offering these blocking tools, with the commission suspecting that service providers would implement them if given the chance. Evidently it was right, with AT&T launching a similar service in December.

Carriers are going to keep getting broader discretion to fight spam calls, too. Just yesterday, the FCC passed a proposal that would let phone companies block some spoofed robocalls. And FCC chairman Ajit Pai has indicated that he intends to keep expanding that toolkit, since robocalls are the top consumer complaint the commission receives each year.

T-Mobile says its service works by comparing phone numbers to a list of “tens of thousands” of known scammers. The database is constantly updated, the company says, by analyzing call patterns. So it sounds like T-Mobile might catch on to new scam numbers if it notices a bunch of subscribers immediately hanging up on a number they’ve never contacted before.

The service is being offered for free to all T-Mobile customers, though there are some slight differences in when subscribers will get it. On April 5th, the warnings will start rolling out to subscribers on T-Mobile’s One plan. They’ll expand to other T-Mobile customers eventually, but there’s no timing yet on prepaid or MetroPCS subscribers. And all customers will have to actively enable the blocking feature, which they can do by dialing #662# after that date.

Update March 24th, 1:38PM ET: The paragraph above has been updated to clarify T-Mobile’s rollout plans, with the company saying scam blocking will eventually reach all customers.