Hello,
Have you verified the problem is not with the analog side of your system? I had similar noise issues when I first setup my system and it turned out to be a wire in my house going to one of my phones. I heard tapping and intermittent static.
The echo is probably on the digital side, ask them to check which port your Sipura is using on the outbound side. I found my router box would not support the port allocation they needed for robust communications, so I re-arranged my network and now it is rock solid with no issues.
Crossed calls, sounds like the phone system in CA somewhere?