IP-to-IP H.323 IOS gateway calls between two remotely located Cisco CallManagers fail in one direction. The called party IP phone rings at the target location, but silence is heard when answered. The calling party continues to hear ringback. The call is cleared down after a few rings with cause code 47
This is the call flow in this scenario:
IP phones -> Cisco CallManager (A) -> Gateway (A) -> IP cloud -> Gateway (B) -> Cisco CallManager (B) -> IP phones
To resolve this issue, perform these steps:
Open the Cisco CallManager Admin page.
Go to the Gateway Configuration page
Uncheck Wait for Far End H.245 Terminal Capability Set.
This step enables Cisco CallManager to send the Terminal Capability Set (TCS) without waiting for the other side. This field applies only to H.323 devices. (By default, this box is checked to specify that Cisco CallManager must initiate capabilities exchange. This check box specifies that the Cisco CallManager must receive the far-end H.245 TCS before it sends its H.245 TCS.)
Note: Also, check to see if the H323 FastStart Inbound parameter is enabled for the H.323 gateway on the Service Parameters page. (This parameter allows the endpoints to establish a basic point-to-point call with as few as one round-trip message exchange, and enables immediate media stream delivery upon call connection.)