If your cell phone rings, and you're not going to answer it, hit the cancel/send-to-voicemail button.
If you want to answer it, but you want to conduct your conversation somewhere more discreet, answer the phone, "Hi, this is <your name>. Can you hold on for a moment?" And then walk somewhere more discreet and talk away.
Do not let your phone ring until it goes to voice mail. Do not let your phone ring loudly throughout a sea of cubicles until you've reached a location which you think is not disturbing to your office mates. If you do that, you've already disturbed your office mates
Especially if you've got your phone set to that annoying default Nokia ringtone.