How to enable Progress Indicator IE on ALERTING with an inbound ISDN call


In some ISDN trunk voice applications, there is a requirement to play voice to a caller prior to the call being completely established. An example might be to say "to avoid being charged for this call please hangup now" prior to sending a CONNECT (when charging begins). Some ISDN switches require a Progress Indicator Information Element (IE) with the initial ALERTING message in order to allocate a voice path ahead of receiving a CONNECT message.


Some ISDN switches do not allocate a voice path until after a call has been connected i.e. not until after a CONNECT has been received and call charging has begun.


Initial configuration of E1 Euro and QSIG ISDN trunks

Diagnosing the problem

Unable to hear voice played by an ISDN application until after an inbound call has been answered.

Resolving the problem

A new System Parameter has been implemented in the Trunk Interface group named Progress Indicator description value.

This parameter has a default value of -1 but otherwise has an integer value in the range of 0 to 255 corresponding to the hex value to be transmitted (as the Progress Description value, byte 4) in the Progress Indicator IE sent with the ALERTING message for an inbound call. The default value of -1 disables the sending of Progress Indicator.

Support for sending Progress Indicator has been added for E1 Euro ISDN and E1 QSIG only and is available in fileset dirTalk.ISDN.Euro-ISDN version onwards.

Note that the new system parameter is independent of the generic (field and lab only) Channel Group parameter Connect Voice Channel Before Answer. Both need to be set for voice to be heard before connect.