You made it pretty clear that you are referring to Bangladesh. Let's look at the status quo. Why do people want VoIP? Because the cost of ordinary telephone calls is too high. VoIP is cheaper. But why?
It's not because of technology: TDM telephony can be even cheaper, in terms of bandwidth, than VoIP; it doesn't waste space on headers, and while the switches tend to be a bit more costly, that's in the noise, on a per-minute basis. No, it's because VoIP is not taxed the same way. It's a different business model. Regular phone calls are still governed by the principles of the 1850s International Telegraph Union, which worked pretty well into the 1980s. But now that fiber optic undersea cables are more common and their price has fallen, the fixed cost of terminating a phone call exceeds the international transit cost. And so any form of evasion becomes attractive.
Compare Bangladesh with its neighbor India. There, the cost of landing a phone call is low. So wholesale rates from the US to India are well under US$0.10/minute. Bangladesh costs 2-4 times as much. Dhaka isn't all that far from Kolkata, so it's not really a distance issue. The Bangladeshi authorities are trying to milk too much from the cash cow. Look at what low cost calling has done for India. They've taken over the world's call center business. Sure, the telco gets less per minute, but they more than make it up in volume, and even if they don't, the country as a whole has benefitted. Does Bangladesh tax roads? After all, they're valuable, so if somebody brings his rice to market on a road, should the road operator extract a share of the cargo? Yet people still let "telecom" work that way. It should be seen as vital national infrastructure, a way to participate in the world economy, not a luxury to be taxed like Scotch Whisky or BMWs.
You're going way wrong here, because what you're doing is replicating a monopoly, so as to tax it. But if you tax VoIP (where "tax" may be a fee to the telephone company, not the general fund, but enforced by government power), then what's the point? Current VoIP is a tax evasion scheme, and a valuable one at that, because the tax is so harmful. So your scheme will result in avoidance too. The Internet is famous for routing around blockages, which sometimes are merely administrative schemes and fees, not real circuit failures.
DPI should be a crime everywhere; it's basically wiretapping. Of course the telcos like it, but it can only be used in a monopoly environment, since otherwise traffic will route around it. And if you're going to set up a monopoly -- or try to -- then why should VoIP be treated differently from TDM? That's picking favorites in the technology world. Do you have different import duties on Windows PCs vs. Macs or Linux systems?
The proper price for terminating a call into a country should be the cost of carrying it from the point of termination to the destination, perhaps subject to the same percentage tax as all calls (i.e., not technologically discriminatory, and not high enough to encourage mass evasion -- think VAT, not luxury tax). Operators could then focus on minimizing costs and maximizing quality, not evading taxes or setting up costly toll booths.
The Internet is NOT technology. It's a business model. The PSTN is a business model which is losing out to the Internet. The technology is a side show, and should be allowed to evolve, not be locked in by some master plan that forever locks in what was thought of as state of the art in 2000 or so.
(I am the Telecom Policy columnist for TMCnet, so you can see some of my essays on related topics there.)