5.8.8. Line With Loop Start Signaling and Straightforward Direct Inward
Dialing (DID) 
There are customers who have a PTT that cannot provide them with DID
because of a lack of resources. The feature described here for main station
interfaces (loop start signaling) can help (if the general numbering
plan allows it).
Naturally you can also use the feature for connecting external systems
with the OpenScape 4000 (see also TC-SL link).