mode codes" as said before,is the way that u have to communicate between the voicemail and the PBX if u don't have a link. It is the less desireable way of connecting due to the slowness and loss of features when using mode codes instead of a link. If u have a link to your voicemail, then u don't need to know anything about mode codes. If u have mode codes, consider upgrading to a link. If u have a older prologic , then u will have to upgrade. If u have most other Definitys, just wiring and software changes will usually get u to a better integration.