Exclamation [Q] [AOSP] Broken telephone feature

Guys, everytime I flashed an AOSP based Rom like Carbon on my hltexx, nobody can call me and I also can't call someone else. If I dial a number, the connection seems to break instantly but my mom told me, she got my calls but didn't hear anything. I'm also unable to change or view the access points.
Stock based Roms like Omega work correctly.

WTF am I doing wrong?

Additional Infos:
>SMS and Internet work.
>3G because my provider (Drei, Austria) is retarded.
>Android 4.4.2
>restoring an old EFS backup didn't solve the problem
>APN seems to be correct