Please let me know if you find out ;) I've been
trying to find RSTS/E
to go on my PDP-11, actually.
You can download the RSTS/E V7 distribution from
http://simh.trailing-edge.com/kits/rstsv7gen.tar.Z although you might
have to have to jump through hoops to get it into a media you can
install on your PDP-11.
Of course, there are others these days, including a
built in "PHONE"
command.
Well, phone is OK for a couple of people chatting, but it's not suited
for a whole pile of users who may want to talk together or seperately,
sending private messages, etc. My "Talk" program uses a Conference
metaphor to allow users to join conferences, and send messages to all
users in that conference, or send "private" messages to a specific user.
It also has a "moderated" conference feature which allows one person to
control who can and can't speak in the conference -- good for panel
discussions, etc.