We have those same phones in our office for the last several years...

This has been pretty normal from the beginning with these at our office where we never used Voipo services until early last year...

We did use Vonage pretty much from the start with them but I seem to remember them doing it with Qwest also.

If you have one you might put a dsl filter straight out of the ATA and see if it goes away. Those phones communicate with each other in the same spectrum that a dsl modem does with the head end. If the ATA is generating noise in that spectrum it could be inducing this... The filter should then stop that.

Just make sure that you do not filter the line between the phones.

Most hardware and office stores stores carry DSL filters in their telco stuff...