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Answering machine apps
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F-Droid is an installable catalogue of FOSS (Free and Open Source Software) applications for the Android platform. The client makes it easy to browse, install, and keep track of updates on your device.
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Its similar but not requiring operator.
Someone calls me. If am not answering within 20 seconds, the app answers the call itself. It records the meesage. Once am available, I listen to the messages. NO Need for operator to record the message.
But as you said, due to restrictions this wont be easy.