Jeff205, yours is an interesting situation because you say when you remove either line, the hum goes away. That sounds less like a power supply issue and more like a poorly designed phone to me, although I hate to blame the phone because it could be something else entirely.
The thought that popped into my head when I read your last message was "ground loop." You sometimes hear it in analog sound systems where you have two different sources on different inputs and if you plug both in you get hum out of the speakers, but not if you disconnect either one (and it's a big reason why optical cabling has become so popular). The thing is, though, that a two line phone should not be susceptible to that — even if you had two commercial phone lines, there's no guarantee they are taking the same path through the building, or even coming from the same central office! So if you only get the problem when both phone lines are connected, then yes, it could be a bad phone. Would reversing the polarity of one of the two lines help? Possibly, if the hum is additive, but then again it could make it worse.
Anyway, if you don't get the hum when the commercial line isn't connected, then it's most likely NOT a bad power supply on the OBi. But since I'm not an OBi engineer, nor an engineer of the company that made your phone or your cable provider for that matter, there's really not much more I can say about that.