Hi I have opened a TAC service request for the problem and we have found a solution for the problem. The problem is caused by CUCM because it tries to provide the busy tone with the Annunciator which results in an ISDN Alerting message with Inband Info instead of a Disconnect message with cause "user busy". If you configure a Media Resource Group List/Media Resource Group that does not contain an Annunciator and assign it to the Gateway config in CUCM, the Disconnect message with "user busy" will be sent out immediately and you should hear the busy tone on your mobile. Extract of the mail from TAC: "I have checked the logs, and I could see the call coming in from the PSTN and the calling number is recognized as a Remote Destination. Once the CallManager finds the destination device is busy it will tell the CellProxy about that and then try to invoke annunciator to play the busy tone : 11:35:06.496 |CellProxy(    298): ccDisconnReq.cv = 17 set MobilityEvent = MOBILITY_EVENT_PLAY_ANNOUNCEMENT - active_CcDisconnReq |1,100,13,13587.4^10.1.10.8^* The Annunciator is allocated to play busy tone for Remote Destination parties and when CUCM detects that the calling party is part of Remote Destination Profile, it does consider as an "internal" calling party and play the busy tone through the Annunciator. If you try to call the same busy phone from the internal IP Phone extension, the Annunciator will here play the busy tone directly to the IP Phone. This is why we see different behavior as compared to other calls from PSTN. Can you please configure the MRGL associated with the H.323 Gateway so that it does not contain any Annunciator resource and see if it fixes the issue? By doing this, we will play directly Busy Tone through the H.323 gateway and the Disconnect (User Busy) message." Regards Stefan ... View more