[Users] Registering ok, but nothing else

[Ronald Wiplinger]

It is my first try, .... forgive me if I ask too complicated.

I have installed openser 1.1 and started it. openserctl moni shows me 
that three phones could register.
When I try to call from one phone to the other, the phone will ring, but 
when I pickup nothing happens.
In my first set-up all phones are on the internal network, connected to 
the internal ethernet port.
Firewall cannot be a problem therefore.

I followed the OpenSER Installation Notes.
What do I miss?
How can I make more tests / see more (sothat I do not need to ask here 
too much)???

bye

Ronald