There are carriers, who offer trunk lines with H.323 protocol. The equipment used by these carriers typically does not support media renegotiation. This must be taken into when doing the configuration.
Normally with VOIP protocols Media data (Voice RTP) is sent from one endpoint directly to the other endpoint regardless which route the signaling for the call has taken. If one endpoint changes, for example because a local transfer of the call, the media needs to be renegotiated so that the far endpoint is sending the media data to the new endpoint.
This media renenotiation if quite often not supported by H.323 carriers.
The problem can be solved by eliminating the need for media renegotiation end to end, even if the local endpoint changes. This is done by two mechanisms: Media Relay and Exclusive Coder
Media Relay means that media data is not sent from one endpoint directly to the other endpoint, but is routed thru the gateway. So for the remote endpoint the address where the media is sent to does not change even if the local endpoint changes. It is always the the gateway.
If the local endpoint changes and media renegotiation happens, it could be that a new coder is negotiated. This again would mean that end to end media renegotiation is needed. To avoid this an exclusive coder has to be configured.
To configure the H.323 trunk two GW definitions (e.g. GW1 and GW2) are needed in the Gateway. One GW to connect to the carrier, the other GW to connect to the local Trunk object of the PBX. To connect to the local Trunk object the IP Address 127.0.0.1 should be used. For both GWs the same exclusive coder has to be configured.
Two routes have to be configured from GW1 to GW2 and from GW2 to GW1.
If media relay is turned on for a give call depends on the Addresses used for the registration of the signaling interfacaes. To turn on media relay for calls from GW1 to GW2 and for calls from GW2 to GW1, the local IP network and 127.0.0.1 have to be configured as Private Networks under IP/Settings.
This way any call between GW1 and GW2 is between a private network and a non-private network, which means media relay is truned on.
If T.38 fax shall be used to the carrier, the Enable T.38 checkmark has to be set for GW1 and GW2.