Thank you very much for your answer.
I did not know anything about webRTC and User Manager, so now I am trying to fix my poor knowledge !
I deleted all the users present in User Management, without prior removing the enable webRTC flag.
Some 99 extensions went away, some other are still present (to be more precise: the extensions bound to extensions still existing are still there; the removed ones were bound to deleted extensions)
So now I am trying to use WebRTC, which seems to me having a very good potential. With Inernet Explorer I have no phone on the upper right; with Mozilla firefox I have it, but when I press the call button it does not do anything; same behaviour in chrome
anyway: this topic can be considered closed and well answered.
If I will need further assistance, I will open a new more specific thread
thanks,
Andrea