Maybe it's just me, but your post is very confusing. You indicate you're having an echo issue with the landline connected to the OBi, but then talk about speed tests and sufficient bandwidth. What does speed and bandwidth have to do with a landline?
You can bypass the OBi by either dialing only '#' or by removing power from the OBi. This will connect the telephone that's attached to the OBi's PHONE Port directly to the landline that's attached to the OBi's LINE Port. If you still have echo, the OBi is not involved.
That's all I can think of without more and clearer information.