talk (software)

talk is a Unix text chat program, originally allowing messaging only between the users logged on to one multi-user computer—but later extended to allow chat to users on other systems.

Although largely superseded by IRC and other modern systems, it is still included with most Unix-like systems today, including Linux,[1] BSD systems[2] and macOS.[3]

No Wikipedia page, but there's also PHONE:

https://marc.vos.net/books/vms/help/phone/

Invokes the OpenVMS Phone utility (PHONE). The Phone utility is designed to simulate some of the features of actual telephone communications. You can use the Phone utility to communicate with other users on your system or with any other system connected to your system by DECnet for OpenVMS. To invoke the Phone utility, enter the PHONE command at the DCL prompt and press RETURN.

I've used both on VMS.