DuckCall is designed to be added onto the current telephone call relay filtering system. This addition to the system mimics an answering machine.
The telephone system acts as a receiver and sends out 1s* (one second) of static to the sender. The system then waits up to 1.75s* for a response. If there is no response (noise) within the 1.75s the call can be presumed to be a Robocall and disconnected. If a sound signal is recognized within the 1.75s then the call should be sent through to the intended receiver.
If the incoming call is digital and the telephone system is digital, a digital-audio-compression program could be utilized to shorten the static sending time and the response time. See http://www.ece.jcu.edu.au/subjects/ee3700/resources/Digital_Audio_Compression.pdf
- The time (1s and 1.75s) may need to be adjusted.