Not quite true.
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)