And right there is the rub. The iPod Touch. It's basically an iPad sub-mini, without the option for a cellular radio.
And what I hear from some die hard iPod owners is that they would like an iPhone Plus-sized Touch. But I don't think that will ever happen. Mainly because the market for iPods isn't that large, but more importantly your suggestion of using a much cheaper Touch as a voip device, is exactly why Apple would not want to risk cannibalizing their flagship iPhone sales.
In fact I don't see the iPod Touch ever getting another refresh, and I expect it to be quietly discontinued sometime after the 7S is released.
However, what if Apple starts offering the same kind of data plans for iPhones they offer for iPads? Then iPod users can pick whatever current iPhone models suit their budget and tech requirements, and then chose to activate a voice plan (with a number), a data plan (without), or leave it on wifi. Heck it would be cheaper for Apple to offer a cellular radio-less iPhone to get the price more affordable for Touch customers, than to continue to develop and build a separate device. And there's no reason not to take that approach, especially with carriers coming around on all fronts. Apple still gets their money, and customers have a much more versital device, and are more likely to give carriers some money at some point than never.