2N VoiceBlue Enterprise - User Manual - Manual (Page 71)

Insight into VoIP Gateway 5-9
Since incoming calls from GSM networks can only be routed to
VoIP lines in the gateway it is impossible to connect a call back to
the GSM network in this type of routing. This, however, does not
mean that this operation cannot be executed on another routing
level, e.g. in a PBX.
If the VoIP telephones used are equipped with the Calling Line
Identification Presentation (CLIP) function, either the internal
number of the gateway output line used for the incoming GSM
call, or the CLIP obtained from the GSM network can be
displayed. This VoIP line parameter is set at the first option by
default. To make the gateway convey a public number to the VoIP
telephone it is necessary to ensure that all routing elements be able
to process the CLIP correctly in the opposite direction.