When I had a symbian phone, things were so easy. I used IVCM (interactive voice call master) to handle incoming calls.
This app is near to perfection, it can distinguish callers (in address book, in some whitelist, in a blacklist, unknown callers and, last but not least, private numbers)
For any of those you can define a suitable treatment, invoke the answering machine after xx seconds, play a notification then hang up, forward the call to another number, play the busy tone, and more options.
Furthermore you can schedule the behaviour, so the phone switches to answering machine at once during the night, let only friends reach you in the evening etc...
Outgoing messages can be personalised by address group, country (=different language!), friends, private number callers etc..
This is a call to the Android developer community. Please build us an app with a similar usability, picking things up where IVCM stopped.
I will be your first customer and prepared to pay well for it!