When answering the telephone, greet the caller with a simple and polite "hello". Try not to start your verbal interaction with something abrupt like "yes", "yeah" "go" or "who's this". (yes, I have been on the other end of a call that was answered with this last one!) Along the same line, this is usually not the time to take a stab at stand-up
Speak clearly and with a smile on your face. You might not think that your facial expression matters when the caller can't see your face but our voices can unintentionally relay our emotions, good or bad. You wouldn't greet someone face to face with a scowl or frown and you shouldn't do it when answering the phone either.