The Cisco ATA186 supports G.729, G.729A, G.729AB2, G.723.1, G.711a-law and G.711µ-law codecs.
The problem is if you are using both analog ports and G.729, the second port is limited to using G.711.
Although this is documented in the datasheet(s), sometimes you need to learn from experience. This is an import consideration if deploying analog devices at sites served by limited bandwidth WAN links. You CAN associate new regions and device pools with only the second port to set G.711 as the codec between regions, but you need to be bandwidth cognizant.