This object specifies the storage type for this conceptual
row. If this conceptual row represents a system built-in
tone plan, the value of this object is readOnly (5).
If this conceptual row represents an user defined tone
plan, the value of this object is nonVolatile (3).
The built-in tone plans will be added by the system
automatically at the system start-up. They can not be
added, deleted or modified by user.