DEV Community: Muhammad Umer Mustufa The latest articles on DEV Community by Muhammad Umer Mustufa (@umer_mustufa_dc812f61edb3). https://dev.to/umer_mustufa_dc812f61edb3 https://media2.dev.to/dynamic/image/width=90,height=90,fit=cover,gravity=auto,format=auto/https:%2F%2Fdev-to-uploads.s3.us-east-2.amazonaws.com%2Fuploads%2Fuser%2Fprofile_image%2F4116271%2Fc9583210-2d58-40a5-af7e-6ca9c0635b8a.png DEV Community: Muhammad Umer Mustufa https://dev.to/umer_mustufa_dc812f61edb3 en Building an On-Device Spam Call Blocker for Android Without Harvesting User Contacts Muhammad Umer Mustufa Tue, 08 Sep 2026 18:13:03 +0000 https://dev.to/umer_mustufa_dc812f61edb3/building-an-on-device-spam-call-blocker-for-android-without-harvesting-user-contacts-3n14 https://dev.to/umer_mustufa_dc812f61edb3/building-an-on-device-spam-call-blocker-for-android-without-harvesting-user-contacts-3n14 <p>Some spam-blocking apps come with a privacy trade-off: to build their lookup databases, they ask for access to your contacts and, in some cases, upload contact data to shared registries.</p> <p>We built <a href="https://getcallro.com" rel="noopener noreferrer">Callro</a> on a different premise: modern Android phones have enough local computing power to make the blocking decision on the device itself.</p> <h3> How On-Device Call Screening Works </h3> <p>Instead of matching caller IDs against a remote contact database, Callro uses Android's native <code>CallScreeningService</code> API combined with a 26-layer local analysis engine (the Gauntlet):</p> <ul> <li> <strong>Carrier verification:</strong> reads the carrier's STIR/SHAKEN verification status (Passed, Failed, or Unverified) as reported by Android.</li> <li> <strong>Additional screening filters:</strong> further on-device checks score each call before Callro decides whether to reject it.</li> <li> <strong>Privacy:</strong> contacts, call logs, and audio never leave the device.</li> </ul> <p>When a call is identified as spam, Callro rejects it through Android's <code>CallScreeningService</code> before the device begins ringing.</p> <p>Check out our technical showcase on <a href="https://github.com/umustufa174-wq/callro-android-spam-blocker" rel="noopener noreferrer">GitHub</a> or visit <a href="https://getcallro.com" rel="noopener noreferrer">Callro</a> to learn more.</p> android security privacy mobile