This object specifies the valid file name stored on
the media gateway's harddisk.
The tone plan file can be transferred to the media gateway
via FTP text file transfer mechanism.
When creating a conceptual row, the tone plan file must
be transferred to the media gateway already.
If the entry is a built-in tone plan predefined in system,
then this object will have the value of 'BUILT-IN'.
Other provision-able tone plan files are not allowed to have
the string 'BUILT-IN' as their names.