Many PBX have an automatic forwarding facility, in order to forward call to the actual extension (local or external calls) after a number of seconds.(or a given number of ringbacks).
When calling in the call will be forwarded to the VMX after e.g. 15 seconds.
Many PBX can be programmed to let calls go direct to the voice mail, when the extension is busy.
All these described facilities are connected to the PBX rather than the VMX system.
///doktor