"Excuse me, but I wonder if you really want other people to overhear your conversation."
"I beg your pardon, but I was wondering if you were aware that you can be heard in the next room."
"It would just be so great if you could maybe speak a little more softly."
"It would really help me a lot if you could lower the volume just a bit."
"Oh, hey, I didn't mean to interrupt, but I was just thinking it sure is hard to get people to hear you on the other end of the phone, isn't it?" (And then most reasonable people would say, "Oh, am I speaking too loud. I'm sorry.")
In summary, you can try various combinations of the phrases suggested here. Put the pieces together any way that suits you, but I think you'll find that using exaggerated politeness and showing some empathy for the loud person's situation (even if you don't feel it's really warranted) can work wonders. Engage the person with respect and compassion (even if that person is not showing those qualities to you), and the good example you set will most often get a good response. (And be prepared to maintain your composure and politeness if the person responds with belligerence, so that you don't escalate the situation.)