I am finally able to call in both directions, :), the final problem was the STUN which is needed in my case; without STUN the phone registers with its private IP.
Unfortunately the forward is not working as yet. I'm not sure if I should open a new thread for that or not.
Here is the description:
Cisco phone is configured to forward all calls to a cell phone
calling the Cisco phone redirects to the cell phone, but it's either
- not ringing, instead I get the "switched off behaviour", which is voice-mail in this case
- ringing once, then goes to voice-mail.
If voice-mail is not activated, the message is the "The phone.. is currently switched off".
My ISP says the call get redirected correctly to the cell number.