(1) See 
voicemail.
(2) (
Virtual 
Machine)  One instance of an operating system and applications in a virtualized environment.  See 
virtual machine.
(3) (
Virtual 
Machine)  A runtime engine for a development platform.  Examples are the Java Virtual Machine and the .NET Common Language Runtime (CLR).  See 
runtime engine.
(4) (
Virtual 
Machine)  The first virtualized OS for IBM mainframes.  VM runs multiple IBM operating systems of any type, each with their own applications.  Originally developed by a separate research arm of the company, VM later became mainstream in the 1970s.  In 2000, IBM introduced z/VM for the 64-bit z/Architecture.
VM-CP and CMS
VM-Control Program (VM-CP) is the "hypervisor" that provides the virtual machine functionality.  Conversational Monitor System (CMS) is a compact guest operating system that is widely used with VM to provide interaction for one user.  Each user communicates with CMS in a separate virtual machine in the computer.  See 
CP/CMS and 
virtual machine.