In How Loud Is Too Loud? we learn that the Loud Talker is the number one pet peeve of office workers. The Loud Talker is closely followed by the Person With Annoying Cell Phone Ring and the Person Who Insists on Using Speaker Phone.
The solution to these pet peeves? Actually opening your own mouth to say something (shocking, I know):
Do that by saying what's bothering you and explaining the effect it has on your work. Then ask the person to either lower his or her voice or take personal calls out of the office. Then, ask if that's OK, to make sure the other person understands and will comply. Pachter offers a more specific example: "Hey, you might not realize--I hear your conversation and it's distracting to me. I'm having a hard time working. Could you please lower your voice? OK?"Works for me.