Yep, I agree and vote for an answering machine. I assume it should be a simple coding for someone familiar with the architecture. 'recaller' anyway picks up and can record any voice as well as incoming calls automatically. Just add an 1 minute of playback and then start recording .....
Why not just pay for a provider's voicemail ? You'll be paying for incoming calls if you set up voice mail on your tablet. So, it will end up costing you about the same. Well it depends on number of calls you get.