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).
AD-DA numbers... don't see the problem
Quote from article: "The audio chip in the Neo1973 only has 2 A/D and 3 D/A. This means that the phone cannot play (stereo?) mp3 while answering the phone for you."
Telephone calls are mono, so in order for the phone to have answering machine capability, it only needs to have one D/A channel available (for the OGM), and one A/D available (for recording the incoming call). If the phone is playing a stereo MP3, that will take up two D/A channels. Given that the above states the Neo1973 has '3' D/A [channels], I don't see why it shouldn't be able to play MP3s and have answerphone functionality.