It's considered rude in Belgium, at least when I commuted there by train. People would leave the wagon to take a call. Maybe it's changed in the past few years. There were also signs that it's prohibited to call in the trains.
Most trains however have a space [0] at the end of each car where you enter and exit. That space can also have a toilet, bike storage, various (locked) train controls, sometimes even a few seats [1] where you can sit if your call will be a while.
0: https://www.cornwallrailwaysociety.org.uk/uploads/7/6/8/3/76...
1: https://www.modernrailways.com/sites/modernrailways/files/im...