hmmmm.. I'm thinking it could be an Avaya thing too...
I saw this on a site running version 3.0.59 I think it was, where 3 different systems,
1 X 406V2, 2 X 403, all with VCM's linked via LAN in 3 different buildings.
The 403 sites used the LAN/ H323 to access the ISDN lines on the 406, so all local lines were in the main office.
it seemed to only occur when calling from the 403 sites across the SCN to the ISDN lines (with Telstra NZ) to Vodafone NZ GSM Mobile phones.
ie if a user on the 403 just dialed out to a mobile even. they just got dead air.
We did receive an alerting message in the trace, but no actual ringing tone in the ear piece.
With Telstra here in NZ they query whether all the progress, ringing, busy tones etc are provided by the Avaya / Customer provided equipment etc. or whether they should provide it at the exchange.
Sorry not much more help, but something odd is going on...