There's usually some space between train coaches (I think it's called a gangway?) or at the end of a coach (a vestibule) that is separated from the seats area by doors on both sides. You can take your call there.
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...