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The 3CX "free" version is really only for testing and perhaps home or very small business use. The intention, after all, is that you purchase a licensed version. Since G729 requires a licence itself, I'm sure your only option is to upgrade to paid.
Be aware thet the use of G.729 essentially cuts the simultaneous call ability of the paid licenses in half so if you have a 16 SC license running 729 it functions as an 8SC and so on.
Ian Joyce, 3CX Distribution Channel Manager 888VoIP / American Technologies, LLC
If you have the PRO Version, you would set up two codec’s.
G729 G711
In the above order, the PRO Version includes 8 x G729 Codec Licences and 3CX Licensed for 16 Concurrent Calls total.
If you have G729 as the default codec to be used, once the full 8 have been used, it will then start using G711 up to the maximum of 16 concurrent calls.
So that would give you 8 Calls at G729 and 8 Calls at G711 (16 Total)