The command to communicate with another user is called TALK.  Its
argument is the device name of the terminal you want to talk to, e.g.,
	TALK TTY123
The command will fail unless the specified terminal is in monitor
mode (i.e., waiting for a monitor command, as opposed to program input)
and is at the left margin (the user must not be in the middle of typing
a command).  When you are in a talk ring, what you type goes only to
the terminals in the ring, not to the monitor or a user program.  To
leave the talk ring, type <call> (control-C from Teletypes).

It is considered antisocial to use the TALK command to establish
communication with strangers.  A better way is the SEND command, which
will send a message to a user even if he is running a program, but
does not interfere with his work.  Type "HELP SEND" for more info.