Personally, I think this problem falls completely outside the scope of Google. He wants people calling a non-Google Voice number to get to his Google Voice voicemail. This is a feature that has to be provided by the network, because it needs to work when your phone is off. T-Mobile does not provide this feature unless you pay them money. He doesn't want to pay them money, so he doesn't get the feature.
It's an annoying arbitrary limitation (because T-Mobile's network supports this just fine; call my "real" T-Mobile number and you get my Google Voice voicemail), but it's not Google's fault. It's just how cell phone service works in the US; it's based around subsidized devices or really cheap featureless devices. Nobody has a featureful unsubsidized smartphone, so it's not a case that the providers handle. They disable features on the cheap subsidized phones because they want to upsell you to a subsidized phone, guaranteeing revenue for 2 years.
Anyway, if the author is setting up a new number for this phone, he can avoid the whole problem by only telling the new number to Google. Everyone else can call his Google Voice number directly, and then it doesn't matter where his real voicemail forwards to. (I stopped telling everyone my real number about 6 months ago, and now it gets no calls.)
Sadly, short of moving to another country, I don't think there's any way the author can get what he wants. (AT&T doesn't even allow the possibility of data use unless you have a multi-year contract.)
Edit: Since the author makes the inevitable iPhone comparison, it's worth pointing out that "Buying an unlocked iPhone is an impossible experience."