5.8.8.1. Feature Description (User Interface) 
The 'external' user calls the connection number. When there
is a call on the line, the OpenScape 4000 generates an ANSWER to the
CO and applies the dial tone. The external user can now dial into the
system. The codes that are possible are the same as for 'normal'
DID.
IMPORTANT:
In this application, the assessment begins with
the answer (application of the dial tone).
The feature described functions only with DTMF apparatuses because
the DID is operated with the DTMF signaling method. This means that the
external user must dial with a DTMF device or use an acoustic coupler.