I tried to use Skype and VoIP on T-Mobile
When you make call after half second you almost can't hear anything it feels like you don't have enough speed but everything else works well.
So telecom - blocks hard VoIP Skype
Vodafone - officially doesn't allow but it works there
O2 - officially allows
Eplus - officially blocks but it works well
For VoIP use eplus or O2 or their mvno Blau, fonic etc.
Alex