Have you tried turning it off and on again?
- Solutionselected by EneaN
What do you hear when you try to call yourself on your cell phone? If busy signal: Have you perhaps activated the “Silence before the telephone” service?
see also here (it’s already one Older post, but apparently the behavior changed with the introduction of IP telephony - I can’t understand it myself because I haven’t had a “landline” Anschluss for years)