Text based synchronous communication tool for Linux?

I have faint memories on a communication tool that I think have used
on a DEC VAX under VMS in the 1980's. Unlike the common modern tools
like chat systems or SMS exchange on mobile phones it didn't operate
line-wise or message-wise, but rather (synchronous) character-wise.
And I think it was a tool usable only to connect people on the same
system (but I'm not sure about that). I thought its name would have
been 'talk' but searching the web led me to a document "VMS Phone
Utility Manual"[*] which describes such a tool (it's actually called
'phone') and it seems to be what I still remember about the tool I'm
looking for.

Some characteristics I'd prefer - not all supported by the 'phone'
utility - are that it's
 * text-oriented (preferably with Unicode support), fast (no GUI)
 * instantly/synchronously exchanging any typed characters
    * optionally: switching modes (instant/character-wise, line-wise)
 * more than two persons can communicate
 * works across distributed [Unix-]systems

Is there such a tool (free of charge and open source) available for
Linux? (Or something that comes close?)

Janis

[*]
http://bitsavers.informatik.uni-stuttgart.de/pdf/dec/vax/vms/5.0/AA-LA08A-TE_VMS_5.0_Phone_Utility_Manual_198804.pdf