What first comes to mind is the original use of “Can you hear me now?!” as in the “reception wasn’t good so I moved over here hoping that you can hear me NOW!”
Next what comes to mind is that the background noise is making it tough to hear.
But these are legitimate excuses for people talking loudly in PRIVATE settings. My feeling is that most people who are so rude as to talk loudly in a public location are, unless there is a true emergency situation that they’re so upset about that they are frozen and can’t move to a more private location, too self-absorbed to be self-conscious or concerned with others.
I think a large part of the problem is that cell use has become so prevalent, pervasive, & constant that people just make & receive calls without considering their surroundings (I can’t count how many times cells phones start ringing loudly during movies & church service!)
In order not to chance a confrontation with someone that might escalate, you might want to speak with someone else, such as a waiter or restaurant manager, who can talk with the offending person with some, for lack of a better word, “authority” as well as professional tact.