Who's There? has an Android app to combat spam calls and telemarketers:
We value your privacy!
Our apps do not collect or share any data from your phone.
To be able to block calls, the apps need permissions to
- Receive info about the phone's state to know that the phone is ringing
- Read the phone's call log to be able to see the phone number of the incoming call
- Read the contact list to be able to not block the user's contacts