Basically you need a transcoder where you have a requirement to run G729 over the WAN, to a device that does not support g729.
What this means in practice is that if you want to make a call from an IP Phone to and IP Phone, you do not need a transcoder - the phones support G729 natively.
If you want to make a call from a gateawy to an IP Phone using G729, you do not need a transcoder. The gateway will use more DSP, so you should have more DSP capacity than PSTN channels (e.g. if you have a remote site with a T1, a PVDM-32 would allow a good number of calls to route over the WAN). The phone supports g729 just fine.
If you want to make a call from a remote gateway or IP Phone, to (for example) a voicemail server at your CUCM site that does not support G729, then you need a transcoder. The transcoder would need to be at the CUCM site, and the media from the call woudl route via the transcoder which would decode the g729 and send it on to the voicemail server as G711.
Other examples of services that may not support G729 are the CUCM conference bridge, or UCCX.
So you need to determine what endpoints you have on each site, and whether they support G729 natively.
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