Hi all.
At the end the problem appears to be in having "voice real-time-config" disabled in the controller:
#show voice real-time-config
Configure Real-Time Analysis
----------------------------
Parameter Value
--------- -----
Real-Time Analysis of voice calls Disabled
You can enable it (that must be done in the Master controller, the change will propagate to the local controller) with
#configure t
(config) #voice real-time-config
(Configure Real-Time Analysis) #config-enable
#show voice real-time-config
Configure Real-Time Analysis
----------------------------
Parameter Value
--------- -----
Real-Time Analysis of voice calls Enabled
With this the calls starts to be classified also as "Fair" and "Poor" and less frequently as "Unknown".
Regards.