Even if the GXW4104s are simple devices their configuration can be tricky. Routing is non-intuitive and the answer supervision settings are not very well documentes.
Luckily I've configured a lot of these devices and I can help you to set up yours. The configuration should take a couple of hours including all the line tests, which are necessary to achieve an echo free setup.
Some background about me: I'm a consultant with more than 10 years of experience in VoIP solutions, having worked on a variety of projects, from small branch office PBXs to large mission critical redundant VoIP routers.