10.23.2. Feature Description 
The external interfaces to the Off Premises Station (OPS) shall be
T1 interface (TMDN-H) and a line interface (SLMA). Message Waiting Indication
via lamp shall not be supported on an OPS line (port), due to what is
deemed to be hazardous voltage requirements.
The introduction of the TMDN board is not covered in this document
as it is a separate feature .