I don’t seem to be charged for Scam Shield on my account. I’ve had one unknown number call and not leave a message over the last month, which is a far cry from the 20–40 spam calls per day some people report.
<https://www.t-mobile.com/benefits/scam-shield>
The free tier seems to identify but not block likely scam calls. T-Mobile's website skews sales-heavy, and doesn't feature much technical information.
The premium features appear to be app-dependent, and may not work on feature phones.