This article answers why. Some snippets:
"VOIP or Voice Over IP is a technology that enables telephone companies to transmit high numbers of phone calls all over the world very cheaply. [..] Creative entrepreneurs have popped up everywhere setting up VOIP internet exchanges throughout Bangladesh that carry a huge volume of international phone calls into Bangladesh using the Internet. These exchanges usually consist of computer servers, gateways, and modems.
These VOIP exchanges carried as much 80% of the total international telephone traffic but were highly illegal. [..]
The new military backed administration has been on a very big anti-corruption campaign against the graft and illegality of the past that Bangladeshi’s have put up with for far too long. Hundreds of illegal businesses, shops and slums have been bulldozed and destroyed. The same has happened to the VOIP internet exchanges that have been making a great living from their illegal internet telephone services.
They have been selling international telephone call capacity to foreign phone card companies who have also benefited greatly by their very low rates and margins.
Closing down these internet exchanges has sent the telephone network and capacity in to complete turmoil in Bangladesh. You now have millions of callers trying to make the same volume of calls on 20% of the capacity that existed previously – there is less and less capacity available all the time."