The phone operators are scared shitless that people will run VOIP over their smartphones and so get to a much lower effective rate per minute than what the operators charge at the moment.

Typically a VOIP stream is between 2400 and 9600 baud, or 300 to 1200 bytes per second. At E0.25 cts per MB for instance that would make their voice tariffs look ridiculous and people would switch en-masse to VOIP.

They can't really win this one, either people will not use their data networks and they end up in front of the anti-competitive agencies (OPTA etc) or they lose a pile of money on their voice side.

Sucks to be them.