See if I can understand your question correctly.
Call scenario is that A calls B via H323 GW and actually call will be forwarded to C so A can connect to C without involving H323 GW
If this is the case it can be by using TBCT
Cisco IOS Tcl IVR and VoiceXML Application Guide - 12.3(14)T and later - Configuring Telephony Call-Redirect Features […
I also saw some service providers can provide this feature in SIP trunk as well. (using SIP REFER)
Tcl is needed (in both cases) to provide a command to request for this type of transfer. The service provider (external PSTN switch) has to provide the feature to take care of this type of transfer.