guys, do whatever the **** is congruent with your personality!
Personally, I don't answer my phone if I'm busy. That means studying, most times I'm in class, if I'm having any sort of conversation with someone and I'm not expecting an important call, etc. This comes from the way I was raised probably -- my dad refused to get caller ID, a cordless phone, or an answering machine. His rationale? "If it's important for me to answer, I'll run to the phone. If it's important for them and I don't answer, they'll call back."
I'm a guy who doesn't like the idea of being "connected" 24/7. Most people who know me recognize this. Some hot girl called me, and I was too busy because I got in my zone and I'm painting some really cool stuff now? Who cares?
On the other hand, a buddy of mine is one of those down-for-anything-anytime type people. Always ready to stop and have a chat with anyone he knows (helps that he has the greatest memory, he can strike up a conversation instantly about some tiny detail he remembers from talking to you once, 4 months ago.) And he always answers his phone to have a chat no matter what he's doing (usual answer? "oh hey Julia/John/Dave/whoever, I was just talking with my friend Maria/Amy/Ari, do you know him/her? Great guy/girl.") Guy gets his, no doubt. He's always at shows or parties, always wanting to talk. Why wouldn't he answer the phone? He wants to talk to _you_, whoever _you_ are.
Finally, one of the best DJs I knew would be out doing a freelance racy photoshoot for lesbians on tuesday, working at the wine bar a few nights a week, and on saturday he'd be in the mood for staying home and making some banana bread. If some girl called, and his phone was on him, he'd usually answer, talk to her about complete BS "oh I was just thinking about pirates actually," and then bang her after she'd help him with banana bread.
My point here is, different people with different styles. Do what you do... But don't fake it.