It's based on the Unity conversation. It's not based on anything external if that's what you're asking (i.e. it's not copying an existing competitive conversation).
The Unity conversation was designed loosly on ISO standards back around 1997 - it caried some residual assumptions from part of the old Repartee voice mail system and has, of course, mutated as the system introduced new features and capabilities.
Just a side note - Unity 4.0(5) and later offers the option of a customizable conversation (to a degree) for subscribers that lets you assign key maps to functions for top level subscriber menus, message playback and after message menu portions of the conversation. Since everyone things their favorite conversation is the "obvious world standard" that everyone should agree with, we went this route to try and make it possible for folks to work it into their existing user base with the minimum of pain.
