I haven't found 1 deaf person that has been offended by ringless voicemail...not 1.
The sound of the ring of the phone isn't the determining factor whether or not you're connected to the service it is intended to provide. Your service is activated and provided once you contract with the telecom provider. Court case at least a couple years down the road will bear this fact out. Gawk scored a major win by being the first out the gate with this and owning the patent, I believe.