Nope I don't think it's possible. Maybe with "H2O" since they run on AT&T's network, but I wanted the the same thing and after hours of research I couldn't find any answers, so i declared it impossible
Nope. Its sad world. Google Voice app has it for its app along with YouMail, but no native support. People have been trying since the iPhone first came out, and still, not one single working tweak.