There are hardly any payphones left these days, but before cellphones came along, they were quite useful. It always fascinated me that restaurants provided a bank of payphones in a relatively private hallway near the restrooms.
Now suppose you needed to go to the bathroom, but you hated to admit that to the people you were dining with. You could say, "I need to make a phone call," and you could disappear, none the wiser.
Even better, suppose you wanted to make a phone call, but you didn't want anybody to know what you were doing. (Use our imagination here.) You could say, "Excuse me, I need to visit the bathroom," and you could disappear, none the wiser.