If you are not getting "off-hook" when one phone is directly plugged in to the OBi, then the OBi is not bad.
Is there any sign of dampness, verdigris, corrosion, fungus etc in any of the wall sockets? That can give a varying resistance between the two wires. Sometimes that will pass enough current to appear as off-hook. Sometimes the brief spurt of AC ringing voltage can temporarily 'burn' that off but the symptoms will reappear sooner or later.
What exactly do you mean by 'quality of the voice is very bad'? Is that happening even when only one phone is plugged directly into the OBi?