OSTN » OSTel

I authorized wifi and 3g connection but to no avail: I can't hear my voice in the echo.
Things are a bit more complex though: first of all, if I try to use csip simple via 3g (with ostel) it does not work It registers itself but I can't hear anything. It works with any other provider.
On the other hand, if I set up my 3g connection to be shared via wifi tethering with another device, this latter one is perfectly able to call via csip and ostel! I don't understand why, since its connection goes through mine in 3g!!!!!
I also tried another way: I connected to another phone who shares its 3g connection with me via wifi and in this case my phone gets the 4 letters confirmation code! So what to do? What's the issue that stops my phone from connecting directly via 3g? Obviously I have authorized 3g connections....