I have the same issue since i have MIUI on my HD2. I don't understand why i have this issue, sometimes call work, sometime he doesn't. Sometimes i have big noises, sometimes i can't hear the person who called me...
And i have turned on the third-party access. So i need help!
EmryX, first you should mention which Radio do you use. mention if you use any Theme, I would like to recommend you the following:
1) fresh install last version of Miui. (format your SD card)
2) Do not install any programs.
3) Do not customize (Theme).
check it for 1 day or 2 days, and see.
I hope this will help to locate the problem.