Phone does not connect although signal is good

Hi folks,
have my phone since september and usually it works fine.
But sometimes phoning does not work. The icon says, I have a good signal, the net connection works, but when I try to make a call, i get immediately “call stopped” or similar. It is also impossible to call me (it’ said I’m out of reach). A while later, sometimes at the same place, it works again, without any changes.

Any ideas?

kind regards,

Interesting. Which network mode are you running? I noticed that when I have 2G & 3G enabled, I sometimes seem to loose connectivity for a short moment without the icon changing. When I have 3G only, I can see the icon changing, shortly, and have a sharp decrease, e.g., in speed (or voice quality is abyssmal).
No problem if 2G only (+ WiFi) is on.

[Edith says I should mention that the 3G network around here is sub-optimal. Especially inside steel-enforced concrete buildings like the one I’m working in.]

Hard to say what it depends on. Position? Building? The icon said the signal was strong and the internet connection worked, just no phone connection.
Anyway, it did not happen for the last weeks, without any changes in configuration.
I have 3G on. Everything else is still in default position.
I write when it happens again…