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.
Article ID:1966 Network Address Translation (NAT) Settings for Lines on
PAP2T and SPA3102 Phone Adapters Objective The objective of this
document is to configure Network Address Translation (NAT) settings for
the lines of communication on the PAP2T and SP...
Article ID:2742 Speed Dial Settings on SPA100 Series Objective Speed
Dial Buttons are often used to make the process of calling regularly
used contacts quicker and with one button. This option is used in place
of dialing the entire phone number in. This h...
Article ID:2676 Firmware Upgrade on SPA112 and SPA122 Objective To
upgrade the firmware a specific file must be downloaded from the
Internet and uploaded to the device. Upgrading your devices can be very
useful. Firmware upgrades can fix software bugs, im...