gitlab.com
Yet Another Call Blocker is an Android application for blocking unwanted calls. Its GitLab project page lists a README, changelog and contribution guide and states it is licensed under the GNU AGPLv3.
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f-droid.org
A simple call blocking application that uses a crowdsourced phone number database. It has an offline database, a local blacklist with wildcard support, and optional automatic blocking of calls with a negative rating. Database updates use third-party servers and do not send user-identifiable information except the queried number. The app is still under development.