Sorry,
I left out the ACK in my signaling sequence. The ACK arrives at the GW after the GW has sent the DISCONNECT message to the ISDN subscriber.
Thanks Manuel
UA GW ISDN INVITE -> | <- 100 Try | TX -> SETUP | RX <- CALL_PROC | RX <- ALERTING <- 183 Sess | | RX <- CONNECT | TX -> CONNECT_ACK <- 200 OK | Milliseconds later ! | TX -> DISCONNECT ACK -> | | RX <- RELEASE | TX -> RELEASE_COMP <- BYE | 200 OK -> |