rheijmer,
Yes there is a way to do it, but if you system is not setup to do it already then it would take some programming to change it. You would also need to know who wants to allow this feature to happen to their phone and who doesn't.
There are two flags on a phone that work in concert to get this function to happen. Handsfree and Ring Intercom Always.
This turns the feature on for those that want to allow it to happen.
Go to System > Phone-Related Information > Flags > Handsfree: Yes
This allows the phone calling to use the Handsfree feature if it is turned on.
Go to System > Phone-Related Information > Flags > Ring Intercom Always: No
Another issue is if any of the System Forwarding paths have an Call Type IC to go somewhere then the Handsfree won't work since the phone needs to ring in order for System Forwarding to work.
Thanks,
TE