This evening, we tried registering with his cell phone and that worked. It's difficult for me to conceive why Google would consider a true-blue twisted-pair account with an "AT&T" RBOC -- one which has been in service longer than Google has been in existence -- to be suspect, while accepting a cheap pay-as-you-go cell plan without reservation. I also wish the error message could more clearly explain the problem, not so much for me, but for those without access to resources like this forum, who must truly be befuddled by such a failure. But I suppose I shouldn't look a gift horse in the mouth. Steve, thanks for the response.
The other thing worth pointing out from this adventure is that Google verifying ownership (or at least access) to a forwarding number (by requiring you to enter a few digits when called), is quite different from Google validating the number (ensuring it meets GV's litmus tests for being a number valid for use as a forwarding number which can be linked to your GV number). The latter step is not performed until you actually attempt to link the forwarding number with the GV number you select.
BTW, does this mean that numbers assigned to VoIP services will no longer pass GV's more stringent validity checks? (Does anyone know?)