1) Turn phone off, it immediately goes to voice mail, if you can't resist answering the phone, this is a good method.
2) Ignore the phone if it is turned on, there is no law that you must answer a phone that rings.
3) Answer phone, with Can't talk right now, can I call you back?
I like being out of contact by not having a cell phone, because I know that some people will expect that the phone is always turned on, always answered, and that talking to them is the most important thing in your life.
Once upon a time I worked shift work, if on nights, before I went to bed I would turn the volume off and unplug the phone from the back of the answering machine. When I got up I checked the messages, and returned those that left messages and were important. Funny though how many people would not leave a message and then call after I got up with, "I have been trying to call you all day, but you didn't answer the phone" and the only thing you think is, "why didn't you leave a message?"