Just calling into the number you provide, I can navigate their IVR system well (I own two OBi 110 units, both seem to be OK with DTMF) , and I have pretty much suggested setting from OBi (method: auto, fixed duration)... When you said Cox was broken, do you mean their system cannot recognize your digits at all? or account didn't get through?
Also, you might try to call the Cox through your Gmail, and see if that gives a better experience ...