What you need to do is set-up a PBX system.
A PBX is kind of like a router in a network.
It takes one VoIP account and splits it up / shares it between other accounts and routes the calls however you choose.
This means you can have both the ATA and ET registered with the VoIP service at the same time without any difficulties and can have complete control over what happens on incoming and outgoing calls.
NCH currently offers a software PBX solution for free called Axon, it should solve the problems you are encountering.