Cell Phones
The main problem with mobile phones is the "mobile" element: people had no such issues when dealing with stationary phones, which compelled them to find a discreet time and location to have a telephone conversation. With cell phones, an array of obnoxious behavior has arisen.
Primarily, answering a phone in the presence of others suggests that they are less important than the person calling. This may sometimes be true (when you are expecting an important call) - but that should be a very rare exception (there are few things that cannot wait a few minutes, or even a few hours).
Also, consider that your conversation is private, between yourself and the other person, and remove yourself to an environment where you will not disturb others.