Hello,
Placing the SPA122 between the ISP and your main router could be a very good or a very bad idea.
The good part is that there will be no "middle man" between it and the internet insuring that routing issues with your calls will be kept to a minimum.You won't have to configure any port forwarding for the calls or anything so it is the best way to use your ATA.
The issue is that the built in router inside the SPA122 is very minimalistic and not too powerful. If you will have several users in the inside of the network connected to the E3000 streaming music and videos via youtube or maybe netflix then most likely you will have issues with traffic becoming slow.
I guess it depends on your specific situation, please let us know.