This is extremely bad. In Portugal I don't cross the road if the person is using a phone, unless they fully stop of course. Yet this is a problem so big that even people walking cross roads with their faces glued to their phones. It happened to me more than once almost getting into an accident, both while walking or driving because the other party is distracted by a smartphone instead of paying attention to driving o…
What is the big deal with phone use at a red light? It will, at worst, hold up traffic slightly.
If it's something frivolous then it can wait. If it's important then are you really able to compartmentalize it? Will you be able to put the conversation out of your mind, or ghost your chat partner as soon as traffic starts moving?