Update: drgeoff, the ChannelRxGain change worked, I played with it and had to actually decrease, not increase, the number from default 5 to 2 and now AA registers the key press and the phone rings. Thank you so much for pointing me in the right direction.
Now, a new problem I just figured out occurs, OOMA voicemail does not work (not related to the ChannelRxGain change as it doesn't work with the default settings either). The phone continues to ring for about 1 min instead of OOMA voicemail to answer after 5 rings, then the call drops. If I turn on the phone's voicemail, it picks up after 5 rings, but it would be nice to be able to use OOMA's voice mail for the additional features... Any idea on how to fix this one? Thank you.