I got sometimes spam calls without a shown number. It would be nice to play a message like "I don't speak to people with hidden number" or something else, without ringing.
Don't telecoms providers normally provide voicemail?
Not quite sure why an answering machine app would be useful, given that most mobile phone service providers (at least, in the UK) include voicemail by default. So if someone calls and you don't pick up (e.g. because you have the phone on silent, and you don't hear it), then after a certain number of rings the call is re-routed to your phone company's voicemail service, on which you can usually record your own custom OGM (OutGoing Message).