H323 gateway is a non-survivable endpoint. This means an active call will drop if any call manager goes down. This default behavior of an H323 gateway can be changed by playing with few settings at the CUCM and gateway.
voice service voip
Allow TCP Keepalive for H323 = False
Allow Peer to Preserve Call = True
Restart CCM service of PUB
Make an inbound call from PSTN to the phone.
During the call, stop the CCM service at Sub but your call will not drop!
This is what you will see at the gateway:
Issue debug on H323 Gateway – debug h225 events
Jun 3 21:09:07.052: h323chan_chn_process_read_socket: fd=3 of type CONNECTED has data
Jun 3 21:09:07.052: h323chan_recvdata: Connection lost fd=3
Jun 3 21:09:07.052: h323chan_close: TCP connection from fd=3 closed
Jun 3 21:09:07.052: h323chan_chn_process_read_socket: fd=2 of type CONNECTED has data
Jun 3 21:09:07.052: h323chan_recvdata: Connection lost fd=2
Jun 3 21:09:07.056: %CCH323-6-CALL_PRESERVED: cch323_h225_handle_conn_loss: H.323 call preserved due to socket closure or error, Call Id = 4589, fd = 2
h323chan_chn_close: Calls Exist on socketfd=2 Owner
Jun 3 21:09:07.056: h323chan_close: TCP connection from fd=2 closed