In London, subway cars are either completely separated and you can't travel between them, or it's just one long continuous tube, there are no vestibules. You don't get much signal underground either way, so it's rare to see people on a call.

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...