Hi Alessandro,
Have you tried playing with the Auto Line Select setting in the phone configuration? This setting comes into play with how the display of secondary calls to the phone are shown. Have a look;
\\Look at the first page of the Phone configuration down towards the bottom @ Auto Line Select
Product Specific Configuration
Disable Speakerphone
Disable Speakerphone and Headset
Forwarding Delay* ------EnabledDisabled
PC Port* ------EnabledDisabled
Settings Access* -- ---EnabledDisabledRestricted
Gratuitous ARP* ------DisabledEnabled
PC Voice VLAN Access* ------DisabledEnabled
Auto Line Select* ------DisabledEnabled
Hope this helps!
Rob