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I am currently using Freeswitch in combination with Kamailio to act as a SBC. Freeswitch is bridging the media and acting as the B2BUA. I have an issue that if I proxy media (required so that the PBX behind the SBC can play MOH etc) when I do a transfer the caller stays on hold and the callee hears nothing. The SIP messaging is good as when you hangup one side the other is disconnected.
Interestingly, when I do not proxy the media the transfer process works, however when using hold Freeswitch, because it is involved in the media path, tries to play back MOH. This is not suitable for two reasons; it requires suitable codecs on the SBC and also it means you only have a single MOH file for all customers. It will also, I believe, increase load.
I can't see any way around this but hopefully someone out there can help.